fix(auth): Create account button silently did nothing on invalid input

[implemented.stream (thanabartbb)]

Root cause

The server-side sign-up works. On production (46d66baf) a real test sign-up returned 303 → /chat, wrangler tail showed event: registered, and the test account was deleted afterwards. The problem is on the client: /login relied on the browser's native constraint validation (required, type=email, minLength=10). When the email was invalid or the password was under 10 characters, the browser blocked the submit and showed only a native bubble:

• the bubble is in English, e.g. "Please lengthen this text to 10 characters or more…";
• it is easy to miss, and some in-app or mobile browsers don't show it at all.

No request is sent, so clicking "สร้างบัญชี DevStack" looks like it does nothing. In headless Chromium (iPhone 13, Pixel 7 and desktop), short-password and empty submits sent 0 requests and only set validationMessage.

Ruled out:

• CSP: form-action 'self' covers the POST.
• Middleware/API 404: /api/auth is a known prefix.
• Origin check: passes.
• Rate limit: not hit.
• Overlays: nothing covers the button.
• Hydration errors: no page errors.

Fix

• noValidate, plus a client check that mirrors the server rules (valid email, password 10–200 characters).
• A visible TH/EN role=alert message, and focus moves to the field with the problem.
• A double submit is ignored, and pending resets on bfcache pageshow.

Tests

• New tests/login-form.test.tsx: 4 of its 5 tests fail on the old component.
• Node 20: lint, typecheck, all 372 tests pass.
• Checked in a local browser (iPhone 13): a short password shows the Thai message with 0 POSTs, and valid input sends 1 POST.

Not caused by today's deploys: login-form.tsx and the password route are unchanged since before f5c15746, so a rollback would not help. Not deployed.

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