gui: add File source type to connection tab

- ConnectionConfig: new file_path and delay fields
- ConnectionRow: File page with path input, Browse button, delay
  spinner (0–10 s), and auto-fill of receiver-ID from filename
- ReceiverManager.connect_receiver: FileBackend wired for 'file' type;
  UBX poll timer skipped for file sources
- FileBackend.register_xcvr: fallback to first unoccupied entry when
  name does not match filename pattern

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

backend/file_backend.py

@@ -35,7 +35,12 @@ class FileBackend(ABackend):
                 e['xcvr'] = xcvr
                 xcvr.on_register(self)
                 return
-        raise Exception(f"No log file found for source \"{xcvr.name}\"")
+        for e in self._entries:
+            if e['xcvr'] is None:
+                e['xcvr'] = xcvr
+                xcvr.on_register(self)
+                return
+        raise Exception(f"No available log file entry for \"{xcvr.name}\"")
 
     def connect(self):
         for e in self._entries: