fix: use solid window background on Windows

[StDensity]

Summary

Fixes the Windows window background so Glyph no longer appears transparent when using dark mode. The macOS translucent/glass-style background is preserved, while non-macOS platforms use the solid theme background color.

Type of change

• Feature
• Bug fix
• Refactor (no behavior change)
• Docs / chore
• Other (please describe)

Linked issues

Closes #1

Testing notes

Reviewed the UI background logic in src/App.tsx and confirmed that non-macOS platforms now use bg-background, which maps to the configured light/dark theme colors.

Manual Windows verification still recommended:

1. Run pnpm tauri:dev on Windows.
2. Open Glyph with Ctrl+B.
3. Test both light and dark mode.
4. Confirm the app window has a solid background and other windows are not visible through it.

Pre-merge checklist

• pnpm exec tsc --noEmit passes
• pnpm lint passes
• cd src-tauri && cargo check passes
• cd src-tauri && cargo clippy --all-targets passes (no new warnings)
• Manually tested the affected paths in pnpm tauri:dev
• For UI changes: screenshot or recording attached above
• For new settings: defaults documented and persisted across restart
• For new permissions: capability added to src-tauri/capabilities/default.json

Notes for reviewers