fix(gateway): add API fallbacks and connection stability improvements

- Add api-fallbacks.ts with structured fallback data for 6 missing API endpoints
  - QuickConfig, WorkspaceInfo, UsageStats, PluginStatus, ScheduledTasks, SecurityStatus
  - Graceful degradation when backend returns 404
- Add heartbeat mechanism (30s interval, 3 max missed)
  - Automatic connection keep-alive with ping/pong
  - Triggers reconnect when heartbeats fail
- Improve reconnection strategy
  - Emit 'reconnecting' events for UI feedback
  - Support infinite reconnect mode
- Add ConnectionStatus component
  - Visual indicators for 5 connection states
  - Manual reconnect button when disconnected
  - Compact and full display modes

Diagnosed via Chrome DevTools: WebSocket was working fine, real issue was
404 errors from missing API endpoints being mistaken for connection problems.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src-tauri/tauri.conf.json

@@ -30,6 +30,9 @@
     "resources": [
       "resources/openfang-runtime/"
     ],
+    "externalBin": [
+      "binaries/ov"
+    ],
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",