feat(account): danger zone, restore import, larger API key field on phones

- Danger zone in settings: delete all own data or the account, both with
  password confirmation; the last server admin cannot delete the account
- DELETE /user/data and /user/account; PurgeUserData removes everything a
  user owns in one transaction (others' tasks in shared lists stay theirs),
  also used by admin user deletion
- MindSift import: 'Hinzufügen' or 'Wiederherstellen' (mode=restore, wipes
  own data only after the file is fully validated, confirm checkbox)
- API key name field and button 44 px on phones (flex-1 collapsed them)

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

frontend/src/components/FormSheet.tsx

@@ -9,13 +9,25 @@ interface FormSheetProps {
   onSubmit: (e: React.FormEvent) => void;
   children: React.ReactNode; // the fields
   submitLabel?: string;
+  danger?: boolean; // red submit button for destructive actions
+  busy?: boolean;
   // Desktop only: open next to the click instead of centred
   anchor?: { x: number; y: number } | null;
 }
 
 // Small create/rename dialogs. Phone: bottom sheet directly above the keyboard, full width, large
 // fields. Desktop: centred dialog (or next to the click). Escape and the backdrop close it.
-export const FormSheet: React.FC<FormSheetProps> = ({ title, icon, onClose, onSubmit, children, submitLabel = 'Erstellen', anchor }) => {
+export const FormSheet: React.FC<FormSheetProps> = ({
+  title,
+  icon,
+  onClose,
+  onSubmit,
+  children,
+  submitLabel = 'Erstellen',
+  anchor,
+  danger = false,
+  busy = false,
+}) => {
   const { inset, visibleHeight } = useKeyboardInset();
   useEffect(() => {
     const onKey = (e: KeyboardEvent) => e.key === 'Escape' && onClose();
@@ -61,7 +73,11 @@ export const FormSheet: React.FC<FormSheetProps> = ({ title, icon, onClose, onSu
           <button type="button" onClick={onClose} className="btn-action flex-1 sm:flex-none h-11 sm:h-8 px-3 text-sm sm:text-xs">
             Abbrechen
           </button>
-          <button type="submit" className="btn-primary flex-1 sm:flex-none h-11 sm:h-8 px-4 text-sm sm:text-xs font-semibold">
+          <button
+            type="submit"
+            disabled={busy}
+            className={`${danger ? 'btn-danger' : 'btn-primary'} flex-1 sm:flex-none h-11 sm:h-8 px-4 text-sm sm:text-xs font-semibold disabled:opacity-50`}
+          >
             {submitLabel}
           </button>
         </div>