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)
