fix: move _toggle_offcanvas callback to _compte_shell.py

The callback was defined in compte_admin.py but controlled components
(compte-offcanvas and compte-offcanvas-open) rendered by account_shell()
in _compte_shell.py. Moving the callback to where the components live
improves code organization and reduces coupling.

Closes #73

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

src/pages/compte_admin.py

@@ -206,13 +206,3 @@ def layout(
 )
 def _toggle_delete_modal(_open, _cancel, is_open):
     return not is_open
-
-
-@callback(
-    Output("compte-offcanvas", "is_open"),
-    Input("compte-offcanvas-open", "n_clicks"),
-    State("compte-offcanvas", "is_open"),
-    prevent_initial_call=True,
-)
-def _toggle_offcanvas(_n, is_open):
-    return not is_open