feat(desktop): add automatic game/app activity detection

Add cross-platform process scanning that detects running games and apps,
then broadcasts them as Rich Presence activities via the existing Phase 1
pipeline (activityStore -> WS -> other users).

- games.json dictionary with 26 entries (games, Spotify, VLC, OBS)
- activityDetector module: polls OS process list every 15s via execFile,
  matches against dictionary, emits IPC on state change only
- IPC bridge in main.ts/preload.ts with cleanup on quit
- Frontend activityBridge subscribes to IPC and feeds activityStore
- Supports Windows (tasklist CSV), macOS (ps -c), Linux (ps)

packages/web/src/components/layout/AppLayout.tsx

@@ -31,6 +31,7 @@ import { useWebSocket } from '../../hooks/useWebSocket';
 import { useFederationToasts } from '../../hooks/useFederationToasts';
 import { useLiveKit } from '../../hooks/useLiveKit';
 import { useDeepLinkHandler } from '../../platform/deepLink';
+import { initActivityBridge, teardownActivityBridge } from '../../platform/activityBridge';
 import { useSpaceStore } from '../../stores/spaceStore';
 import { useChatStore } from '../../stores/chatStore';
 import { useUIStore } from '../../stores/uiStore';
@@ -122,6 +123,12 @@ export function AppLayout() {
   // Deep link handler for Electron (backspace:// protocol)
   useDeepLinkHandler();
 
+  // Electron activity detection bridge (game/app process scanning → activityStore)
+  useEffect(() => {
+    initActivityBridge();
+    return () => teardownActivityBridge();
+  }, []);
+
   // Track the last channel we attempted to connect to, to prevent effect loops
   const lastAttemptedRef = React.useRef<string | null>(null);