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>

packages/auth/package.json

@@ -11,7 +11,9 @@
   },
   "scripts": {
     "typecheck": "tsc --noEmit",
-    "lint": "eslint src --max-warnings=0"
+    "lint": "eslint src --max-warnings=0",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@ketopath/db": "workspace:*",