Add native maxLength to headline textarea

Eyebrow already had maxLength as a belt-and-suspenders alongside the
onChange clamp; headline's textarea was missing it. Blog format was
already the default (og format id, label "Blog") — confirmed via a
fresh reload, no change needed there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/og-generator/app/page.tsx

@@ -1004,6 +1004,7 @@ export default function Page() {
                   id="headline"
                   value={headline}
                   onChange={(e) => setHeadline(clampChars(e.target.value, HARD_LIMIT))}
+                  maxLength={HARD_LIMIT}
                   rows={3}
                   className="resize-none rounded-md border border-default bg-surface-100 px-3 py-2 text-sm text-foreground outline-none focus:border-strong"
                   placeholder="Type a blog headline…"