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/activityBridge.ts

@@ -0,0 +1,30 @@
+import type { Activity } from '@backspace/shared';
+import { useActivityStore } from '../stores/activityStore';
+
+let unsubscribe: (() => void) | null = null;
+
+export function initActivityBridge(): void {
+  if (unsubscribe) return; // already initialized
+  if (!window.backspace?.onActivityDetected) return; // not Electron
+
+  // Subscribe to future activity changes from main process
+  unsubscribe = window.backspace.onActivityDetected((activity) => {
+    if (activity) {
+      useActivityStore.getState().pushActivities([activity as Activity]);
+    } else {
+      useActivityStore.getState().pushActivities([]);
+    }
+  });
+
+  // Request current state (handles instance-switch: game was already running)
+  window.backspace.getCurrentActivity?.().then((activity: unknown) => {
+    if (activity) {
+      useActivityStore.getState().pushActivities([activity as Activity]);
+    }
+  }).catch(() => {});
+}
+
+export function teardownActivityBridge(): void {
+  unsubscribe?.();
+  unsubscribe = null;
+}