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>

gui/data_model.py

@@ -52,16 +52,20 @@ class SatelliteData:
 @dataclass
 class ConnectionConfig:
     receiver_id: str
-    conn_type:   str = 'tcp'
-    host:        str = ''
-    port:        int = 8721
-    serial_port: str = ''
-    baud_rate:   int = 115200
+    conn_type:   str   = 'tcp'
+    host:        str   = ''
+    port:        int   = 8721
+    serial_port: str   = ''
+    baud_rate:   int   = 115200
+    file_path:   str   = ''
+    delay:       float = 1.0
 
     def display_str(self) -> str:
         if self.conn_type == 'tcp':
             return f"{self.receiver_id}  [{self.host}:{self.port}]"
-        return f"{self.receiver_id}  [{self.serial_port} @ {self.baud_rate}]"
+        if self.conn_type == 'serial':
+            return f"{self.receiver_id}  [{self.serial_port} @ {self.baud_rate}]"
+        return f"{self.receiver_id}  [file: {os.path.basename(self.file_path)}]"
 
     def to_dict(self):
         return dataclasses.asdict(self)
@@ -148,6 +152,7 @@ class ReceiverManager(QObject):
     def connect_receiver(self, rid: str):
         from listeners import NavSatQtListener, GsaQtListener
         from backend.serial_backend import SerialBackend
+        from backend.file_backend   import FileBackend
 
         config = self._configs.get(rid)
         state  = self._states.get(rid)
@@ -160,8 +165,10 @@ class ReceiverManager(QObject):
             backend = NetworkBackend(
                 [{'name': rid, 'host': config.host, 'port': config.port}]
             )
-        else:
+        elif config.conn_type == 'serial':
             backend = SerialBackend(config.serial_port, config.baud_rate)
+        else:
+            backend = FileBackend([config.file_path], delay=config.delay)
 
         backend.register_xcvr(xcvr)
 
@@ -184,12 +191,12 @@ class ReceiverManager(QObject):
         state.transceiver = xcvr
         state.connected   = True
 
-        # Poll UBX messages every second (receiver only sends on request by default)
-        timer = QTimer(self)
-        timer.setInterval(1000)
-        timer.timeout.connect(lambda r=rid: self._poll_ubx(r))
-        timer.start()
-        state.poll_timer = timer
+        if config.conn_type != 'file':
+            timer = QTimer(self)
+            timer.setInterval(1000)
+            timer.timeout.connect(lambda r=rid: self._poll_ubx(r))
+            timer.start()
+            state.poll_timer = timer
 
         self._push_lru(config)
         self.connection_changed.emit(rid, True)