fix: add switchable liquid glass mode

[davidarny]

Summary

• add Glass as a separate app theme mode instead of keeping liquid glass always enabled
• toggle the native macOS liquid-glass effect from the frontend theme hook via a dedicated Tauri command
• restore solid panel styling for System/Light/Dark and keep the glass shell only for Glass mode
• tighten the panel/nav shell geometry so the border, divider, and hover surfaces do not bleed into each other

Screenshots

Verification

• cargo check
• bun run build
• bun run test --run src/lib/settings.test.ts src/pages/settings.test.tsx src/hooks/app/use-settings-display-actions.test.ts src/App.test.tsx
• bun run test --run src/components/side-nav.test.tsx src/App.test.tsx

Notes

• Skipped bun run test:coverage per request.

[robinebers]

I really like this, thank you @davidarny!

• I think we should make the window a little less wide now
• The gray no longer really works now, and it probably should be either a lighter gray or even white
• Text can rely on the normal/bold to create hierarchy
• Probably need to adjust the remaining UI elements, like the settings and the bars, to appear more glassy

This is amazing progress though, am gonna jump into this a bit.

Would love to collect some feedback, gonna make a post.