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>

sax_file.py

@@ -126,6 +126,19 @@ def open_file():
         messagebox.showwarning("Action Blocked", "Cannot load SAX profiles because 'Sedona home folder' configuration is missing or empty.")
         return
 
+    if app_state.live_mode:
+        answer = messagebox.askyesnocancel(
+            "Device Connected",
+            "A live device connection is active.\n\n"
+            "Disconnect from the device and open a file?\n\n"
+            "Choose Yes to disconnect and open, No to cancel.",
+        )
+        if not answer:
+            return
+        # Import here to avoid circular import at module level
+        from main import do_device_disconnect
+        do_device_disconnect()
+
     file_path = filedialog.askopenfilename(
         title="Select SAX File",
         filetypes=[("SAX Files", "*.sax"), ("XML Files", "*.xml"), ("All Files", "*.*")],