✨ Add showPairingCode endpoint with remote access toggle

[guiyanakuang]

Closes #4095

Summary

• Add GET /sync/showPairingCode server endpoint that navigates the local app to the pairing code screen
• Add enableRemoteShowPairingCode config toggle (default: true) to control whether unpaired devices can trigger the endpoint
• Place the toggle on the pairing code screen so users can disable it after initial pairing
• Implement full event chain: SyncHandler → SyncEvent.ShowPairingCode → SyncResolver → SyncDeviceManager → SyncClientApi
• Optimize pairing code screen spacing to ensure all content (including toggle) is visible without scrolling
• Add i18n key allow_remote_show_pairing_code in all 10 languages

Changed files (36 files)

• Config: AppConfig, DesktopAppConfig — new enableRemoteShowPairingCode property
• Server: SyncRouting, DefaultServerModule, DesktopServerModule — new endpoint with config check
• Token service: AppTokenApi, AppTokenService, DesktopAppTokenService, HeadlessAppTokenService — showPairingCode() + preShowPairingCode()
• Sync chain: SyncHandler, SyncEvent, GeneralSyncHandler, MarketingSyncHandler, SyncResolver, SyncDeviceManager
• Client API: SyncClientApi — showPairingCode() request
• UI: PairingCodeContentView — toggle switch + spacing optimization
• DI: DesktopUiModule, DesktopNetworkModule — wiring
• Error code: StandardErrorCode.REMOTE_SHOW_PAIRING_CODE_DISABLED
• i18n: All 10 locale files
• Test infra: TestAppConfig, TestConfigManager, TestAppTokenService, TestServerModule, TestInstance, SyncTest