feat(desktop): add macOS dock badge for chats with unseen activity

Show a count of chats (root sessions) with unseen activity on the macOS
dock icon. The count is computed in the existing tray snapshot (full
cross-project list, not the capped tray view; a subtask's unseen rolls up
to its root only when subtask notifications are enabled) and pushed to the
main process over the existing desktop_tray_update IPC, which calls
app.setBadgeCount (0 clears it). The badge clears as sessions are marked
seen on window focus.

Add a Dock badge toggle in Appearance settings (default on, persisted,
darwin desktop only), localized across all dictionaries, with a matching
settings-search entry whose availability mirrors the render guard exactly.

packages/ui/src/lib/i18n/messages/zh-TW.settings.ts

@@ -1491,6 +1491,8 @@
   'settings.openchamber.visual.actions.reloadThemes': '重新載入主題',
   'settings.openchamber.visual.field.macVibrancy': '視窗透明度',
   'settings.openchamber.visual.field.macVibrancyHint': '在側邊欄後使用 macOS 原生模糊（vibrancy）。關閉以獲得完全不透明的介面。',
+  'settings.openchamber.visual.field.dockBadge': 'Dock 標記',
+  'settings.openchamber.visual.field.dockBadgeHint': '在 macOS Dock 圖示上顯示有未讀動態的對話數量。',
   'settings.openchamber.visual.actions.saveAndRestart': '儲存並重新啟動',
   'settings.openchamber.visual.actions.restarting': '正在重新啟動…',
   'settings.openchamber.visual.actions.reloadingThemes': '正在重新載入主題...',