Persist values, tighten validation, and a11y on /onboarding

- Server action echoes kitaName, the three module booleans, and the
  Mindest-Verfügbarkeiten value on every error path so failed submits no
  longer wipe what the user typed; OnboardingState gains an `attempt`
  counter that drives a per-field remount key on the client.
- Schema now rejects min=0 when the Notdienst module is active (via
  superRefine) — previously a kita could be created with 0 minimum days,
  which is fachlich sinnlos. When the module is off, 0 stays valid.
- Validation error messages reworded from terse fragments ("Nicht
  negativ.", "Bitte ganze Zahl.") to full sentences.
- Field error <p> elements gain role="alert" + aria-live="polite" so
  screen readers announce them on submit.
- CardTitle now accepts as="h1" and the onboarding page uses it for
  "Willkommen, …" — the page now has a real top-level heading.
- ModuleCheckbox uses aria-labelledby + aria-describedby so the
  accessible name is just the module title; the longer description is
  exposed as secondary info instead of being concatenated into the AN.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/ui/card.tsx

@@ -24,7 +24,7 @@ const CardHeader = React.forwardRef<HTMLDivElement, React.HTMLAttributes<HTMLDiv
 CardHeader.displayName = "CardHeader";
 
 type CardTitleProps = React.HTMLAttributes<HTMLElement> & {
-  as?: "div" | "h2" | "h3" | "h4" | "h5" | "h6";
+  as?: "div" | "h1" | "h2" | "h3" | "h4" | "h5" | "h6";
 };
 
 const CardTitle = React.forwardRef<HTMLElement, CardTitleProps>(