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>

apps/web/e2e/home.spec.ts

@@ -5,7 +5,7 @@ test('home page renders Italian copy and CTAs', async ({ page }) => {
 
   await expect(page).toHaveTitle(/KetoPath/);
   await expect(page.getByRole('heading', { name: 'KetoPath' })).toBeVisible();
-  await expect(page.getByRole('button', { name: 'Inizia gratis' })).toBeVisible();
-  await expect(page.getByRole('button', { name: 'Scopri come funziona' })).toBeVisible();
+  await expect(page.getByRole('link', { name: 'Inizia gratis' })).toBeVisible();
+  await expect(page.getByRole('link', { name: 'Scopri come funziona' })).toBeVisible();
   await expect(page.getByText(/non sostituiscono il parere di un medico/)).toBeVisible();
 });