test(auth): add unit env tests and E2E sign-up/sign-in flow

Unit tests on @ketopath/auth (added to the Vitest workspace):
- readAuthEnv accepts a valid configuration
- rejects secret < 32 chars and non-URL BETTER_AUTH_URL
- preserves Google credentials when both vars are set

Playwright e2e/auth.spec.ts:
- happy path: sign-up creates a user, redirects home with welcome message,
  sign-out clears session, sign-in with the same credentials restores it
- error path: invalid credentials surface a role="alert" message
- each test uses a unique generated email to avoid DB collisions

home.spec.ts: links instead of buttons (CTAs are now next/link).

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

vitest.workspace.ts

@@ -1,3 +1,3 @@
 import { defineWorkspace } from 'vitest/config';
 
-export default defineWorkspace(['./packages/shared']);
+export default defineWorkspace(['./packages/shared', './packages/auth']);