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.

src/network/EnhancedSecureWebRTCManager.js

@@ -12389,6 +12389,16 @@ async processMessage(data) {
         return this.fileTransferSystem.rejectIncomingFile(fileId);
     }
 
+    async getReceivedFileObjectURL(fileId) {
+        if (!this.fileTransferSystem) return null;
+        return this.fileTransferSystem.getObjectURL(fileId);
+    }
+
+    revokeReceivedFileObjectURL(url) {
+        if (!this.fileTransferSystem) return;
+        this.fileTransferSystem.revokeObjectURL(url);
+    }
+
     // ============================================
     // SESSION ACTIVATION HANDLING
     // ============================================