Tasks: Fix Login Window ttk.Frame Background Rendering on macOS

Implementation Tasks

1. [x] Add TFrame background style configuration (image_generator.py:130)

   • Add style.configure('TFrame', background=bg_color) after line 128
   • Ensures all ttk.Frame widgets (including main_frame) have white background
   • Validation: Verify main_frame and all nested frames have white background on macOS

2. [x] Add TLabel background style configuration (image_generator.py:131)

   • Add style.configure('TLabel', background=bg_color) after TFrame configuration
   • Future-proofs against ttk.Label usage
   • Maintains consistency with ImageGeneratorApp styling pattern
   • Validation: Ensure no visual regression; ttk widgets styled consistently

3. [ ] Verify complete UI visibility on macOS

   • Test that title label "登录" is visible
   • Test that username label and input field are visible
   • Test that password label and input field are visible
   • Test that checkboxes remain visible
   • Test that login button is visible
   • Test that error label area is visible
   • Validation: Screenshot comparison - all elements should be visible on white background

4. Cross-platform visual verification

   • Test on macOS (primary target)
   • Test on Windows (ensure no regression)
   • Test on Linux if available (ensure no regression)
   • Validation: UI should render identically across platforms with white background

5. Functional interaction testing

   • Test typing in username field (should be visible)
   • Test typing in password field (should be visible with masked characters)
   • Test clicking login button (should be visible and clickable)
   • Test checkbox interaction
   • Test tab navigation between all fields
   • Validation: All interactions work with visible feedback

Dependencies

• None - tasks can be completed sequentially in order

Notes

• Tasks 1-2 can be done in a single code edit (add 2 lines)
• Tasks 3-5 require manual testing after code changes
• Previous fix (Entry widget bg/fg colors) should remain unchanged