Fix 300s executor shutdown with threading.Event

Replace time.sleep() with threading.Event.wait() in all poll
loops so worker threads exit immediately on shutdown instead of
blocking for up to 500ms per iteration. Fixes the on_unmount
crash (NoMatches from querying removed DOM nodes) by signaling
the event directly rather than iterating child widgets.

Three shutdown paths covered: q key (on_unmount), Ctrl+C
(try/finally in main), and Textual internal shutdown.

tui/src/birdcage_tui/app.py

@@ -6,6 +6,7 @@ device status bar, and five swappable screen panels.
 
 import argparse
 import logging
+import threading
 
 from textual.app import App, ComposeResult
 from textual.binding import Binding
@@ -52,6 +53,7 @@ class BirdcageApp(App):
     firmware_name: str = "g2"
     skip_init: bool = False
     device: object = None
+    shutdown_event: threading.Event = threading.Event()
 
     @property
     def SUB_TITLE(self) -> str:  # noqa: N802
@@ -130,13 +132,8 @@ class BirdcageApp(App):
             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
+        """Signal all worker threads to exit and disconnect the device."""
+        self.shutdown_event.set()
         if self.device and hasattr(self.device, "disconnect"):
             self.device.disconnect()
 
@@ -173,4 +170,11 @@ def main() -> None:
     app.serial_port = args.port
     app.firmware_name = args.firmware
     app.skip_init = args.skip_init
-    app.run()
+    try:
+        app.run()
+    except KeyboardInterrupt:
+        pass
+    finally:
+        app.shutdown_event.set()
+        if app.device and hasattr(app.device, "disconnect"):
+            app.device.disconnect()