fix(parental-lock): tell Enter in the PIN field from a click on Save

Moving on to an empty repeat was decided by document.activeElement in
submit(). WebKit does not focus a clicked button, so a click on Save
with the PIN field focused looked like Enter: focus moved and no error
showed. The advance is now a keydown.enter handler on the PIN field
that prevents the submission, and submit() refuses an empty repeat on
every path.

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

docs/architecture/parental-lock.md

@@ -429,7 +429,8 @@ so a cancelled or refused PIN leaves it showing the real state.
   disabled default button; `submit()` refuses instead, shakes the field
   (no shake under `prefers-reduced-motion: reduce`) and focuses it — except
   that Enter in the PIN field with the repeat still empty only moves focus
-  to the repeat. Set
+  to the repeat (a `keydown.enter` handler: focus cannot tell Enter from a
+  click on Save, since WebKit does not focus a clicked button). Set
   mode marks both inputs `autocomplete="new-password"`. The Electron
   `parental-lock-pin-dialog.e2e.ts` covers the labels and the
   mismatch → fix → save flow.