Fix sweep/scan Stop button and state cleanup

_sweeping/_scanning flags were never reset when workers finished,
leaving the UI stuck in "Stopping..." forever. Both _do_sweep and
_do_scan now use try/finally to always clear state and reset button
styles. Firmware sweep checks the flag after the blocking serial
call returns and discards results if Stop was pressed mid-execution.

tui/src/birdcage_tui/screens/signal.py

@@ -434,6 +434,7 @@ class SignalScreen(Container):
         if device is None:
             return
 
+        try:
             # Check if user forced software mode.
             force_software = self.query_one("#sweep-software-mode", Checkbox).value
 
@@ -447,10 +448,14 @@ class SignalScreen(Container):
                         exc_info=True,
                     )
                     self.app.call_from_thread(
-                    self._set_sweep_status, "Firmware sweep failed -- falling back..."
+                        self._set_sweep_status,
+                        "Firmware sweep failed -- falling back...",
                     )
 
             self._do_sweep_software(device)
+        finally:
+            self._sweeping = False
+            self.app.call_from_thread(self._reset_sweep_buttons)
 
     def _do_sweep_firmware(self, device: DeviceLike) -> None:
         """Firmware-accelerated sweep via azscanwxp (runs in worker thread)."""
@@ -494,6 +499,11 @@ class SignalScreen(Container):
             num_xponders=iterations,
         )
 
+        # Check if Stop was pressed while firmware was executing.
+        if not self._sweeping or shutdown.is_set():
+            self.app.call_from_thread(self._set_sweep_status, "Sweep stopped")
+            return
+
         if not results:
             self.app.call_from_thread(
                 self._set_sweep_status, "Firmware sweep returned no data"
@@ -600,6 +610,11 @@ class SignalScreen(Container):
         self.query_one("#sweep-progress", ProgressBar).update(progress=pct)
         self.query_one("#sweep-status-text", Static).update(status_text)
 
+    def _reset_sweep_buttons(self) -> None:
+        """Restore sweep button styles to idle state (main thread)."""
+        self.query_one("#btn-start-sweep", Button).variant = "primary"
+        self.query_one("#btn-stop-sweep", Button).variant = "default"
+
     # -- Sweep button handlers --
 
     def _handle_sweep_start(self) -> None:
@@ -616,12 +631,16 @@ class SignalScreen(Container):
         self.query_one("#sweep-progress", ProgressBar).update(progress=0)
         self._set_sweep_status("Starting sweep...")
 
+        self.query_one("#btn-start-sweep", Button).variant = "default"
+        self.query_one("#btn-stop-sweep", Button).variant = "warning"
+
         self._sweeping = True
         self._do_sweep()
 
     def _handle_sweep_stop(self) -> None:
         self._sweeping = False
         self._set_sweep_status("Stopping...")
+        self._reset_sweep_buttons()
 
     def _export_sweep_csv(self) -> None:
         if not self._sweep_data:
@@ -647,12 +666,21 @@ class SignalScreen(Container):
     @work(thread=True)
     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
 
+        try:
+            self._do_scan_inner(device)
+        finally:
+            self._scanning = False
+            self.app.call_from_thread(self._reset_scan_buttons)
+
+    def _do_scan_inner(self, device: DeviceLike) -> None:
+        """Inner scan logic (called from _do_scan worker thread)."""
+        worker = get_current_worker()
+        shutdown = self.app.shutdown_event
+
         az_start = self._read_float("scan-az-start", 160.0)
         az_end = self._read_float("scan-az-end", 220.0)
         az_step = self._read_float("scan-az-step", 1.5)
@@ -815,6 +843,11 @@ class SignalScreen(Container):
         self.query_one("#scan-progress", ProgressBar).update(progress=pct)
         self.query_one("#scan-status-text", Static).update(status_text)
 
+    def _reset_scan_buttons(self) -> None:
+        """Restore scan button styles to idle state (main thread)."""
+        self.query_one("#btn-start-scan", Button).variant = "primary"
+        self.query_one("#btn-stop-scan", Button).variant = "default"
+
     # -- Scan button handlers --
 
     def _handle_scan_start(self) -> None:
@@ -831,12 +864,16 @@ class SignalScreen(Container):
         self.query_one("#scan-progress", ProgressBar).update(progress=0)
         self._set_scan_status("Starting scan...")
 
+        self.query_one("#btn-start-scan", Button).variant = "default"
+        self.query_one("#btn-stop-scan", Button).variant = "warning"
+
         self._scanning = True
         self._do_scan()
 
     def _handle_scan_stop(self) -> None:
         self._scanning = False
         self._set_scan_status("Stopping...")
+        self._reset_scan_buttons()
 
     def _export_scan_csv(self) -> None:
         if not self._scan_data: