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/es.settings.ts

@@ -1572,6 +1572,8 @@ export const settingsDict = {
   "settings.openchamber.visual.actions.reloadThemes": "Recargar temas",
   "settings.openchamber.visual.field.macVibrancy": "Transparencia de la ventana",
   "settings.openchamber.visual.field.macVibrancyHint": "Usa el desenfoque nativo de macOS (vibrancy) detrás de la barra lateral. Desactívalo para superficies totalmente sólidas y opacas.",
+  "settings.openchamber.visual.field.dockBadge": "Insignia en el Dock",
+  "settings.openchamber.visual.field.dockBadgeHint": "Muestra en el icono del Dock de macOS el número de chats con actividad sin ver.",
   "settings.openchamber.visual.actions.saveAndRestart": "Guardar y reiniciar",
   "settings.openchamber.visual.actions.restarting": "Reiniciando…",
   "settings.openchamber.visual.actions.reloadingThemes": "Recargando temas...",