release: v4.8.8 file transfer consent fix

Complete the mandatory receiver-consent gate that was wired in the
backend but never connected to the UI callback chain:

- Add the missing onIncomingFileRequest (4th) callback to
  setFileTransferCallbacks in app.jsx — its absence caused
  handleFileTransferStart to auto-reject every incoming file.
- Remove independent callback registration from FileTransferComponent;
  the component was overwriting app-level callbacks on mount and
  nulling all four on unmount, silently breaking progress/received/
  error handlers whenever the panel was hidden.
- Lift pendingIncomingFiles state to the root component so consent
  prompts are shown regardless of panel visibility; auto-open the
  panel on incoming request.
- Add getReceivedFileObjectURL / revokeReceivedFileObjectURL on
  EnhancedSecureWebRTCManager for download buttons in the panel.
- Update file-transfer-ui-cleanup regression test to match the new
  single-owner callback architecture.
- All 14 tests pass; clean production build.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "securebit-chat",
-  "version": "4.8.7",
+  "version": "4.8.8",
   "description": "Secure P2P Communication Application with End-to-End Encryption",
   "main": "index.html",
   "scripts": {