feat(qml): task #13 登录页接 AuthBridge 真 db 认证

LoginScreen.qml wiring 改造：
- appState.login → auth.login (走 DatabaseManager.authenticate / pymysql)
- 删 || "demo" 兜底，密码空时 UI 阻断
- submitting 状态：登录中 username/password/checkbox/button 全 disabled，按钮文字 "登录中…"
- 监听 auth.loginFailed → errorLabel 红字展示后端返回的具体原因
  ("用户名或密码错误" / "无法连接到服务器" 等，DatabaseManager 已统一中文化)
- usernameField 不再硬编码 "chaijin"，改 placeholder

冒烟测试（直接调 Python 测真 db）：
- chaijin/wrongpwd → False，loginFailed 信号"用户名或密码错误"
- chaijin/a160827 → True，loggedIn=True，currentUser='chaijin'
QML 启动登录页渲染正常（task13_login.png 已验证）。

注：
- "记住用户名/密码" checkbox UI 保留但暂未持久化，task #18 系统集成时接 config_util 落盘
- pymysql 同步阻塞主线程最多 5s（connect_timeout），暂不影响 UX；
  若 db 慢成痛点 task #18 时再改 worker 异步

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

@@ -9,15 +9,40 @@ Rectangle {
     color: App.Theme.bgCanvas
     focus: true
 
-    Keys.onReturnPressed: appState.login(usernameField.text, passwordField.text || "demo")
-    Keys.onEnterPressed:  appState.login(usernameField.text, passwordField.text || "demo")
+    // 提交期间禁止重复点击 / 回车
+    property bool submitting: false
+
+    function doLogin() {
+        if (submitting) return
+        if (usernameField.text.length === 0 || passwordField.text.length === 0) {
+            errorLabel.text = "用户名和密码不能为空"
+            return
+        }
+        errorLabel.text = ""
+        submitting = true
+        // AuthBridge.login 是同步（pymysql 5s timeout），主线程会卡一下；
+        // task #18 时若 db 慢需求改异步再说
+        var ok = auth.login(usernameField.text, passwordField.text)
+        submitting = false
+        // 失败时 auth 会发 loginFailed 信号，由下面 Connections 接住
+        // 成功时 auth.loggedInChanged → AppState 转发 → Main.qml StackLayout 切换
+    }
+
+    Keys.onReturnPressed: doLogin()
+    Keys.onEnterPressed:  doLogin()
+
+    Connections {
+        target: auth
+        function onLoginFailed(message) {
+            errorLabel.text = message
+        }
+    }
 
     ColumnLayout {
         anchors.centerIn: parent
         width: 360
         spacing: 0
 
-        // 标题
         Label {
             text: "登录"
             font.family: App.Theme.fontFamily
@@ -37,19 +62,19 @@ Rectangle {
             Layout.bottomMargin: App.Theme.space5
         }
 
-        // 用户名
         CaptionLabel {
             text: "用户名"
             Layout.bottomMargin: App.Theme.space2
         }
         ThemedTextField {
             id: usernameField
-            text: "chaijin"
+            text: ""
+            placeholderText: "请输入用户名"
+            enabled: !root.submitting
             Layout.fillWidth: true
             Layout.bottomMargin: App.Theme.space4
         }
 
-        // 密码
         CaptionLabel {
             text: "密码"
             Layout.bottomMargin: App.Theme.space2
@@ -58,19 +83,21 @@ Rectangle {
             id: passwordField
             echoMode: TextInput.Password
             placeholderText: "••••••••"
+            enabled: !root.submitting
             Layout.fillWidth: true
             Layout.bottomMargin: App.Theme.space2
         }
 
-        // 复选行
         RowLayout {
             spacing: App.Theme.space4
             Layout.fillWidth: true
             Layout.bottomMargin: App.Theme.space5
 
             CheckBox {
+                id: rememberUser
                 text: "记住用户名"
                 checked: true
+                enabled: !root.submitting
                 font.family: App.Theme.fontFamily
                 font.pointSize: App.Theme.fontSm
                 contentItem: Text {
@@ -100,8 +127,10 @@ Rectangle {
             }
 
             CheckBox {
+                id: rememberPassword
                 text: "记住密码"
                 checked: true
+                enabled: !root.submitting
                 font.family: App.Theme.fontFamily
                 font.pointSize: App.Theme.fontSm
                 contentItem: Text {
@@ -133,20 +162,22 @@ Rectangle {
             Item { Layout.fillWidth: true }
         }
 
-        // 登录按钮
         PrimaryButton {
-            text: "登录"
+            text: root.submitting ? "登录中…" : "登录"
+            enabled: !root.submitting
             Layout.fillWidth: true
-            onClicked: appState.login(usernameField.text, passwordField.text || "demo")
+            onClicked: doLogin()
         }
 
         Label {
-            id: hint
+            id: errorLabel
             text: ""
             font.family: App.Theme.fontFamily
             font.pointSize: App.Theme.fontXs
-            color: App.Theme.textTertiary
-            Layout.alignment: Qt.AlignHCenter
+            color: App.Theme.danger
+            wrapMode: Text.Wrap
+            horizontalAlignment: Text.AlignHCenter
+            Layout.fillWidth: true
             Layout.topMargin: App.Theme.space3
         }
     }