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/platform/electron.d.ts

@@ -48,6 +48,10 @@ interface BackspaceElectronAPI {
   getAutoLaunchSettings: () => Promise<{ openAtLogin: boolean; startMinimized: boolean }>;
   setAutoLaunchSettings: (settings: { openAtLogin?: boolean; startMinimized?: boolean }) =>
     Promise<{ openAtLogin: boolean; startMinimized: boolean }>;
+
+  // Activity detection (game/app process scanning)
+  onActivityDetected: (callback: (activity: unknown) => void) => (() => void);
+  getCurrentActivity: () => Promise<unknown>;
 }
 
 interface Window {