feat: add close-to-tray toggle in General settings

[qicaijie99]

Summary

Add an option to minimize to system tray instead of quitting the app when closing the window.

What changed

• Add closeToTray setting to General settings page
• Implement tray icon creation and window hide/show logic in Tauri backend
• Persist setting via settingsStore

How verified

• cd desktop && bun run lint — TypeScript type-check passed
• cd desktop && bun run test — 730 tests passed, 0 failed

Files changed

File	Change
desktop/src/pages/Settings.tsx	Toggle UI in General section
desktop/src-tauri/src/lib.rs	Tray icon and window minimize logic
desktop/src/stores/settingsStore.ts	State persistence
desktop/src/types/settings.ts	closeToTray type
desktop/src/i18n/locales/{en,ja,ko,zh}.ts	Translations