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/screens/scan.py

@@ -7,7 +7,6 @@ raw (az, el, rssi) data for offline analysis.
 
 import csv
 import logging
-import time
 from pathlib import Path
 
 from textual import work
@@ -98,6 +97,7 @@ class ScanScreen(Container):
     def _do_scan(self) -> None:
         """Execute the AZ/EL grid scan in a background thread."""
         worker = get_current_worker()
+        shutdown = self.app.shutdown_event
         device = self._device
         if device is None:
             return
@@ -146,7 +146,7 @@ class ScanScreen(Container):
 
         for _el_idx, el_val in enumerate(el_values):
             for _az_idx, az_val in enumerate(az_values):
-                if not self._scanning or worker.is_cancelled:
+                if not self._scanning or worker.is_cancelled or shutdown.is_set():
                     self.app.call_from_thread(self._set_status, "Scan stopped")
                     return
 
@@ -160,7 +160,7 @@ class ScanScreen(Container):
                     continue
 
                 # Settle time -- let the motor stop and vibrations damp.
-                time.sleep(0.3)
+                shutdown.wait(0.3)
 
                 # Read signal.
                 try: