Fix executor shutdown warning on TUI exit

Poll threads (position at 2 Hz, signal when monitoring) run
while-loops with time.sleep() that never exit on app quit.
Add on_unmount() to PositionScreen, SignalScreen, and BirdcageApp
to clear polling flags and disconnect the device, so worker threads
exit within the sleep interval instead of hitting the 300s timeout.

tui/src/birdcage_tui/app.py

@@ -129,6 +129,17 @@ class BirdcageApp(App):
         if hasattr(screen, "on_show"):
             screen.on_show()
 
+    def on_unmount(self) -> None:
+        """Stop all polling threads and disconnect the device on shutdown."""
+        for mode_key in MODES:
+            screen = self.query_one(f"#{mode_key}")
+            if hasattr(screen, "_polling"):
+                screen._polling = False
+            if hasattr(screen, "_monitoring"):
+                screen._monitoring = False
+        if self.device and hasattr(self.device, "disconnect"):
+            self.device.disconnect()
+
     def action_toggle_dark(self) -> None:
         self.dark = not self.dark
 

tui/src/birdcage_tui/screens/position.py

@@ -87,6 +87,10 @@ class PositionScreen(Container):
             self._polling = True
             self._poll_worker = self._do_position_poll()
 
+    def on_unmount(self) -> None:
+        """Stop polling thread on teardown."""
+        self._polling = False
+
     # ------------------------------------------------------------------
     # Position poll worker
     # ------------------------------------------------------------------

tui/src/birdcage_tui/screens/signal.py

@@ -71,6 +71,10 @@ class SignalScreen(Container):
         """Called when this screen becomes visible."""
         pass  # Monitoring is explicit via Start/Stop buttons.
 
+    def on_unmount(self) -> None:
+        """Stop monitoring thread on teardown."""
+        self._monitoring = False
+
     # ------------------------------------------------------------------
     # Signal poll worker
     # ------------------------------------------------------------------