Add showPairingCode endpoint with remote access toggle

[guiyanakuang]

Summary

Implement server-side GET /sync/showPairingCode endpoint that allows remote devices to navigate the local app to the pairing code screen. Includes a security toggle to prevent unauthorized access.

Changes

• Server endpoint: GET /sync/showPairingCode navigates to pairing code screen and shows main window
• Security toggle: enableRemoteShowPairingCode config (default: true) — checked before handling the request
• Full event chain: SyncHandler → SyncEvent.ShowPairingCode → SyncResolver → SyncDeviceManager → SyncClientApi
• UI toggle: Added to PairingCodeContentView so users can disable remote access after initial pairing
• Layout optimization: Reduced spacing in pairing code screen to ensure all content (including toggle) is visible without scrolling
• i18n: Added allow_remote_show_pairing_code key in all 10 languages