Guard file open and device connect against conflicting state

Opening a file while connected to a device prompts to disconnect first.
Connecting to a device while a file is open prompts to save unsaved
changes, then closes the file before showing the connect dialog.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

@@ -1927,6 +1927,22 @@ _cov_poll_id = None
 
 def do_device_connect():
     """Opens the connect dialog and initiates a device connection."""
+    if app_state.current_file_path and not app_state.live_mode:
+        if app_state.is_dirty:
+            answer = messagebox.askyesnocancel(
+                "Unsaved Changes",
+                "The current file has unsaved changes.\n\n"
+                "Save before connecting to a device?",
+            )
+            if answer is None:
+                return
+            if answer:
+                sax_file.execute_direct_overwrite_save()
+        sax_file.reset_to_startup_state()
+        sax_file.refresh_treeview_from_dom(None)
+        wiresheet.render_components([])
+        app_state.window.title(app_state.base_window_title())
+
     connect_dialog.ConnectDialog(app_state.window, _connect_to_device)