Workspaces Launcher UI: WPF → WinUI Migration

@chatasweetie is already working on this.

Since Jun 16, 2026.

## Summary

Migrate the Workspaces Launcher UI from WPF to WinUI to support PowerToys' broader modernization efforts around Fluent design, theming consistency, accessibility, and long-term maintainability.

This work focuses specifically on the **Workspaces Launcher UI** and does **not** include changes to the Workspaces Editor, workspace configuration model, launch logic, or feature development.

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## Motivation

Migrating this to WinUI improves:

- Consistent Fluent design across PowerToys
- Accessibility support
- Light/Dark theme handling
- Long-term maintainability

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## Scope

### In Scope

- Migrate Workspaces Launcher UI from WPF to WinUI 3
- Recreate existing launcher window UI
- Preserve existing functionality and visual behavior
- Preserve current launch workflow
- Verify accessibility parity
- Verify Light/Dark theme support
- Remove launcher-specific WPF UI assets once migration is complete

### Out of Scope

- Workspaces Editor migration
- Workspace launch engine changes
- New features
- UX redesigns
- Telemetry changes
- Configuration format changes

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## Testing

### Functional

- Launch workspace successfully
- Progress/status updates display correctly
- Error states render correctly
- Window lifecycle behaves as expected
- Multiple launch scenarios function correctly

### Accessibility

- Keyboard-only navigation
- Narrator validation
- Focus management validation
- High contrast support

### Visual

- Light theme
- Dark theme
- Multiple DPI scales
- Multiple monitor configurations

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## Success Criteria

- Workspaces Launcher UI runs entirely on WinUI
- Functional parity maintained
- Accessibility parity maintained or improved
- Light/Dark themes work correctly
- Launcher-specific WPF UI removed
- No regressions introduced