feat: replace notifications with import status in profile dropdown

Remove the notification system entirely and show import progress
directly in the user dropdown menu. Block new imports while one
is already running.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

assets/controllers/import_modal_controller.js

@@ -43,6 +43,7 @@ export default class extends Controller {
             }
 
             this._showFeedback('Import lancé !', false);
+            document.dispatchEvent(new CustomEvent('import:started'));
             setTimeout(() => this.close(), 1500);
         } catch (e) {
             this._showFeedback('Une erreur est survenue.', true);