Disconnect from the device when opening a file or closing the app

Opening a file now drops the live connection once a file is chosen, without
asking. File > Exit and the window close box go through quit_app, which
closes the Sox session properly instead of leaving it for the device to
time out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sax_file.py

@@ -31,6 +31,15 @@ def set_tree_filled_callback(callback):
     _tree_filled = callback
 
 
+_disconnect_device = None
+
+
+def set_disconnect_device_callback(callback):
+    """Registers how to drop the live device session before a file takes its place."""
+    global _disconnect_device
+    _disconnect_device = callback
+
+
 def parse_xml_to_tree(treeview, parent_node, xml_element):
     """Recursively traverses XML elements and adds <comp> tags to the Treeview."""
     for child in xml_element:
@@ -132,19 +141,6 @@ 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", "*.*")],
@@ -152,6 +148,11 @@ def open_file():
     if not file_path:
         return
 
+    # A session is a file or a device, never both. The device goes only once a file has
+    # actually been chosen, so cancelling the dialog leaves the connection as it was.
+    if app_state.device_session is not None and _disconnect_device is not None:
+        _disconnect_device()
+
     canvas = app_state.canvas
     tree = app_state.tree