Add desktop notification bridge for DS Chat Desktop (#49)

Offline members now also get a desktop_notification WS envelope
alongside the existing Web Push send, since Electron has no push
delivery service configured. The client only acts on it when
window.dsDesktop is present and the user's local preference allows it,
so the server needs no awareness of which clients are Electron.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/lib/desktopBridge.ts

@@ -0,0 +1,77 @@
+// #49: the native bridge DS Chat Desktop (a separate Electron wrapper, not
+// this repo) exposes through a context-isolated preload script. Optional on
+// `Window` -- absent entirely in every browser, and even inside the Electron
+// shell a given method may be missing if the wrapper is an older build (see
+// isDesktopNotificationsSupported/isDesktopClickListenerSupported below,
+// each independently feature-tested rather than assumed present together).
+export interface DesktopNotificationRequest {
+  eventId: string
+  roomId: string
+  title: string
+  body: string
+}
+
+declare global {
+  interface Window {
+    dsDesktop?: {
+      setUnreadCount?(unreadRoomCount: number): void
+      showNotification?(notification: DesktopNotificationRequest): void
+      onNotificationClick?(callback: (roomId: string) => void): () => void
+    }
+  }
+}
+
+// Field limits Electron enforces on its side (documented in #49) -- applied
+// here too, defensively, right at the bridge boundary rather than upstream
+// in the shared notification-payload construction (server-side and Web
+// Push have no such constraint; this is specifically the desktop bridge's
+// contract, not a general notification-payload rule).
+const MAX_TITLE_LENGTH = 100
+const MAX_BODY_LENGTH = 500
+const MAX_ID_LENGTH = 128
+
+export function isDesktopNotificationsSupported(): boolean {
+  return typeof window.dsDesktop?.showNotification === 'function'
+}
+
+export function isDesktopClickListenerSupported(): boolean {
+  return typeof window.dsDesktop?.onNotificationClick === 'function'
+}
+
+// No-ops silently if the bridge or this specific method isn't present --
+// callers don't need to guard, matching the rest of this module's
+// capability-detect-per-method philosophy (see #49's "feature-test each
+// bridge method before calling it").
+export function showDesktopNotification(request: DesktopNotificationRequest): void {
+  const show = window.dsDesktop?.showNotification
+  if (!show) return
+  show({
+    eventId: request.eventId.slice(0, MAX_ID_LENGTH),
+    roomId: request.roomId.slice(0, MAX_ID_LENGTH),
+    title: request.title.slice(0, MAX_TITLE_LENGTH),
+    body: request.body.slice(0, MAX_BODY_LENGTH),
+  })
+}
+
+// Returns an unsubscribe function, or undefined if the bridge doesn't
+// support click callbacks at all (older wrapper, or no bridge) -- callers
+// should treat a missing return the same as a no-op cleanup.
+export function onDesktopNotificationClick(callback: (roomId: string) => void): (() => void) | undefined {
+  return window.dsDesktop?.onNotificationClick?.(callback)
+}
+
+const PREFERENCE_KEY = 'ds-chat-desktop-notifications-enabled'
+
+// Purely local -- unlike Web Push, desktop notifications need no server
+// round trip to enable/disable (no subscription row to create/delete), so
+// this is a plain localStorage flag, deliberately decoupled from
+// PushSubscription existence rather than reusing/repurposing it. Defaults
+// to enabled: once running inside the desktop app at all, off-by-default
+// would just mean the common case needs an extra click for no real benefit.
+export function getDesktopNotificationsEnabled(): boolean {
+  return localStorage.getItem(PREFERENCE_KEY) !== 'false'
+}
+
+export function setDesktopNotificationsEnabled(enabled: boolean): void {
+  localStorage.setItem(PREFERENCE_KEY, String(enabled))
+}