feat: add hide Dock icon option in Settings

[xsharp]

Summary

• Add a toggle in Settings > General to hide the app's Dock icon on macOS
• App runs as tray-only when Dock icon is hidden
• Uses tauri::ActivationPolicy for native macOS Dock control
• Setting persists and auto-applies on app startup
• Option only visible on macOS platform

Changes

• Backend: AppSettings.hide_dock field + set_dock_visible Tauri command
• Frontend: Toggle switch in General tab (macOS only) with dock API call
• i18n: Added en-US / zh-CN translations

Test plan

• Open Settings > General on macOS
• Toggle "Hide Dock Icon" on — verify Dock icon disappears
• Toggle off — verify Dock icon reappears
• Restart app with setting enabled — verify Dock icon stays hidden
• Verify option is not shown on non-macOS platforms

Closes #3

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