refactor(editor): remove legacy split-view system and introduce source view lifecycle

* remove toggle_source_view plugin and related command implementations
* drop built-in sibling focus/move commands from core
* simplify EditorsContainer (Paned → Box) and editor initialization
* refactor source view creation to return (scrolled_window, source_view)
* add source view lifecycle events (remove/removed)
* rename GetNewCommandSystemEvent → CreateCommandSystemEvent
* update CommandsController to handle command system creation and cleanup
* ensure command systems are removed with their source views
* improve focus border handling across sibling chains
* clean up imports to use absolute core.* paths
* update keybindings to remove deprecated split navigation commands

user_config/usr/share/app_name/code-key-bindings.json

@@ -42,17 +42,8 @@
         "save_file_as": {
             "released": "<Control><Shift>s"
         },
-        "focus_left_sibling": {
-            "released": "<Control>Page_Up"
-        },
-        "focus_right_sibling": {
-            "released": "<Control>Page_Down"
-        },
-        "move_to_left_sibling": {
-            "released": "<Control><Shift>Up"
-        },
-        "move_to_right_sibling": {
-            "released": "<Control><Shift>Down"
+        "toggle_plugins_ui": {
+            "released": "<Control><Shift>p"
         }
     }
 }