feat(qml): task #15a 款式设计 tab 主体（8 字段 + Prompt 预览 + 生成）

qml_poc/qml/StyleDesignerTab.qml (新):
  - 左侧 360px 卡片：8 字段 ComboBox 表单（jewelry.categories Repeater）
    每个字段头插 "（不选）" 让用户能清空
    onActivated → updateField(category, value) → 触发 reassemble
  - 右侧上半 Prompt 预览：实时调 jewelry.previewPrompt(formData)
    Text 只读展示 + 字数统计
  - 右侧操作行：生成图片 PrimaryButton (调 imageGen.submitTask
    默认 1:1 / 2K / 慢速模式 Pro 模型) + 重置字段 + 状态指示器
  - 右侧下半预览：lastResultPath Image，双击系统查看器打开
  - 监听 imageGen 4 个信号但只对 currentTaskId 匹配的做反应（taskId
    唯一可区分图片生成 tab vs 款式设计 tab，互不冲突）
  - 监听 jewelry.libraryChanged → optionsRepeater.reloadAll
    (task #15b 词库管理对话框触发时刷新 ComboBox model)

MainWindow.qml: 款式设计占位 Item → StyleDesignerTab {}

未做（task #15b）：
  - 词库管理子对话框（每个类别能增删词条）
  - jewelry.addItem / removeItem / resetAll Slot 都已就位，缺 UI

视觉验证：QML_DEBUG_TAB=1 直接进款式设计 tab，8 字段 ComboBox + Prompt
预览（PromptAssembler 在空表单时给 fallback "一款高端精品珠宝戒指设计…
高端珠宝渲染" 52 字）+ 生成按钮全就位，UI 无回归。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

@@ -95,17 +95,7 @@ Rectangle {
 
                 ImageGenTab {}
 
-                // 款式设计 — 占位
-                Item {
-                    Text {
-                        anchors.centerIn: parent
-                        text: "款式设计 tab 内容\n(QML PoC 暂未实现)"
-                        color: App.Theme.textTertiary
-                        font.family: App.Theme.fontFamily
-                        font.pointSize: App.Theme.fontLg
-                        horizontalAlignment: Text.AlignHCenter
-                    }
-                }
+                StyleDesignerTab {}
 
                 HistoryTab {}
             }

+import QtQuick
+import QtQuick.Controls.Basic
+import QtQuick.Layouts
+import "components"
+import "." as App
+
+Item {
+    id: tab
+
+    // ===== 状态 =====
+    // 8 字段表单值（与 jewelry.categories 顺序一致）
+    property var formData: ({
+        "主石形状": "",
+        "主石材质": "",
+        "金属": "",
+        "花头形式": "",
+        "戒臂结构": "",
+        "戒臂处理": "",
+        "辅石镶嵌": "",
+        "特殊元素": "",
+    })
+    property string assembledPrompt: ""
+    property string currentTaskId: ""
+    property string lastResultPath: ""
+    property string statusText: "● 就绪"
+    property color statusColor: App.Theme.success
+
+    function updateField(category, value) {
+        var copy = Object.assign({}, tab.formData)
+        copy[category] = value || ""
+        tab.formData = copy
+        tab.assembledPrompt = jewelry.previewPrompt(copy)
+    }
+
+    function reassemble() {
+        tab.assembledPrompt = jewelry.previewPrompt(tab.formData)
+    }
+
+    function submit() {
+        if (tab.currentTaskId !== "") return
+        if (tab.assembledPrompt.trim().length === 0) {
+            tab.statusText = "● 选几个字段先"
+            tab.statusColor = App.Theme.danger
+            return
+        }
+        try {
+            // submitTask 同步返回 task_id，自己记下来给信号过滤用
+            var taskId = imageGen.submitTask(
+                tab.assembledPrompt,
+                [],            // 款式设计不带参考图
+                "1:1",         // 珠宝出图常用比例
+                "2K",
+                "慢速模式"     // Pro 模型，质量优先
+            )
+            tab.currentTaskId = taskId
+            tab.statusText = "● 已提交"
+            tab.statusColor = App.Theme.accent
+        } catch (e) {
+            tab.statusText = "● " + e
+            tab.statusColor = App.Theme.danger
+        }
+    }
+
+    Component.onCompleted: reassemble()
+
+    Connections {
+        target: jewelry
+        function onLibraryChanged(category) {
+            optionsRepeater.reloadAll()
+        }
+    }
+
+    // 监听生成信号 — 只对自己提交的 task 做反应（taskId 唯一可区分本 tab vs 图片生成 tab）
+    Connections {
+        target: imageGen
+
+        function onTaskProgress(taskId, progress, msg) {
+            if (taskId !== tab.currentTaskId) return
+            tab.statusText = "● " + (msg || "生成中…")
+            tab.statusColor = App.Theme.accent
+        }
+        function onTaskCompleted(taskId, resultPath, prompt, model) {
+            if (taskId !== tab.currentTaskId) return
+            tab.lastResultPath = resultPath
+            tab.currentTaskId = ""
+            tab.statusText = "● 已完成"
+            tab.statusColor = App.Theme.success
+        }
+        function onTaskFailed(taskId, error) {
+            if (taskId !== tab.currentTaskId) return
+            tab.currentTaskId = ""
+            tab.statusText = "● " + (error || "失败")
+            tab.statusColor = App.Theme.danger
+        }
+    }
+
+    RowLayout {
+        anchors.fill: parent
+        anchors.margins: App.Theme.space5
+        spacing: App.Theme.space4
+
+        // ===== 左侧：8 字段表单 =====
+        Card {
+            Layout.preferredWidth: 360
+            Layout.fillHeight: true
+
+            ColumnLayout {
+                anchors.fill: parent
+                anchors.margins: App.Theme.space4
+                spacing: App.Theme.space3
+
+                CaptionLabel {
+                    text: "款式字段"
+                    font.pointSize: App.Theme.fontLg
+                }
+
+                ScrollView {
+                    Layout.fillWidth: true
+                    Layout.fillHeight: true
+                    clip: true
+
+                    ColumnLayout {
+                        width: parent.width
+                        spacing: App.Theme.space3
+
+                        Repeater {
+                            id: optionsRepeater
+                            model: jewelry.categories  // 8 个类别名
+
+                            // task #15b 词库变化时重新拉
+                            function reloadAll() {
+                                // QML 自动重新评估 model:，主动 reset
+                                model = []
+                                model = jewelry.categories
+                            }
+
+                            delegate: ColumnLayout {
+                                Layout.fillWidth: true
+                                spacing: 4
+
+                                CaptionLabel {
+                                    text: modelData
+                                    font.pointSize: App.Theme.fontBase
+                                }
+                                ThemedComboBox {
+                                    Layout.fillWidth: true
+                                    // 头插一个 "（不选）" 让用户能清空字段
+                                    model: ["（不选）"].concat(jewelry.getOptions(modelData))
+                                    onActivated: {
+                                        var v = currentText === "（不选）" ? "" : currentText
+                                        tab.updateField(modelData, v)
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        // ===== 右侧：Prompt 预览 + 生成 + 结果 =====
+        ColumnLayout {
+            Layout.fillWidth: true
+            Layout.fillHeight: true
+            spacing: App.Theme.space4
+
+            // Prompt 预览卡
+            Card {
+                Layout.fillWidth: true
+                Layout.preferredHeight: 200
+
+                ColumnLayout {
+                    anchors.fill: parent
+                    anchors.margins: App.Theme.space4
+                    spacing: App.Theme.space3
+
+                    RowLayout {
+                        spacing: App.Theme.space3
+                        CaptionLabel {
+                            text: "Prompt 预览"
+                            font.pointSize: App.Theme.fontLg
+                        }
+                        Item { Layout.fillWidth: true }
+                        Label {
+                            text: tab.assembledPrompt.length + " 字"
+                            font.family: App.Theme.fontFamily
+                            font.pointSize: App.Theme.fontXs
+                            color: App.Theme.textTertiary
+                        }
+                    }
+
+                    ScrollView {
+                        Layout.fillWidth: true
+                        Layout.fillHeight: true
+
+                        Rectangle {
+                            anchors.fill: parent
+                            color: App.Theme.bgSubtle
+                            radius: App.Theme.radiusMd
+
+                            Text {
+                                anchors.fill: parent
+                                anchors.margins: App.Theme.space3
+                                text: tab.assembledPrompt
+                                color: App.Theme.textPrimary
+                                font.family: App.Theme.fontFamily
+                                font.pointSize: App.Theme.fontSm
+                                wrapMode: Text.Wrap
+                            }
+                        }
+                    }
+                }
+            }
+
+            // 操作行
+            RowLayout {
+                spacing: App.Theme.space3
+                Layout.fillWidth: true
+
+                PrimaryButton {
+                    text: tab.currentTaskId !== "" ? "生成中…" : "生成图片"
+                    enabled: tab.currentTaskId === ""
+                    onClicked: tab.submit()
+                }
+                SecondaryButton {
+                    text: "重置字段"
+                    onClicked: {
+                        var fresh = {}
+                        for (var i = 0; i < jewelry.categories.length; i++) {
+                            fresh[jewelry.categories[i]] = ""
+                        }
+                        tab.formData = fresh
+                        tab.reassemble()
+                    }
+                }
+                Label {
+                    text: tab.statusText
+                    font.family: App.Theme.fontFamily
+                    font.pointSize: App.Theme.fontSm
+                    color: tab.statusColor
+                }
+                Item { Layout.fillWidth: true }
+            }
+
+            // 预览大图
+            Card {
+                Layout.fillWidth: true
+                Layout.fillHeight: true
+                Layout.minimumHeight: 240
+
+                ColumnLayout {
+                    anchors.fill: parent
+                    anchors.margins: App.Theme.space4
+                    spacing: App.Theme.space3
+
+                    CaptionLabel {
+                        text: "预览"
+                        font.pointSize: App.Theme.fontLg
+                    }
+
+                    Rectangle {
+                        Layout.fillWidth: true
+                        Layout.fillHeight: true
+                        color: App.Theme.bgSubtle
+                        radius: App.Theme.radiusMd
+
+                        Image {
+                            anchors.fill: parent
+                            anchors.margins: App.Theme.space3
+                            source: tab.lastResultPath ? "file:///" + tab.lastResultPath : ""
+                            fillMode: Image.PreserveAspectFit
+                            smooth: true
+                            asynchronous: true
+                            cache: false
+                            visible: tab.lastResultPath !== ""
+
+                            MouseArea {
+                                anchors.fill: parent
+                                cursorShape: Qt.PointingHandCursor
+                                onDoubleClicked: {
+                                    if (tab.lastResultPath) {
+                                        Qt.openUrlExternally("file:///" + tab.lastResultPath)
+                                    }
+                                }
+                            }
+                        }
+
+                        Text {
+                            anchors.centerIn: parent
+                            visible: tab.lastResultPath === ""
+                            text: "选择字段后点 \"生成图片\""
+                            color: App.Theme.textTertiary
+                            font.family: App.Theme.fontFamily
+                            font.pointSize: App.Theme.fontSm
+                        }
+                    }
+                }
+            }
+        }
+    }
+}