feat: Kanban touch drag, admins delete backups, bigger share message

- Kanban on phones/tablets: hold a card, drag it onto a card or column, release; the page scrolls at the edges
- Admin settings: database backups can be deleted (DELETE /api/v1/admin/backups/{name})
- Share message: large Liste/Aufgabe buttons, close button, tap outside closes

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

frontend/src/lib/api.ts

@@ -1305,6 +1305,10 @@ export const api = {
     return ensureArray(await request<BackupFile[]>('/api/v1/admin/backups', { method: 'POST' }));
   },
 
+  async adminDeleteBackup(name: string): Promise<BackupFile[]> {
+    return ensureArray(await request<BackupFile[]>(`/api/v1/admin/backups/${encodeURIComponent(name)}`, { method: 'DELETE' }));
+  },
+
   async adminDownloadBackup(name: string): Promise<Blob> {
     return downloadBlob(`/api/v1/admin/backups/${encodeURIComponent(name)}`);
   },