feat: add Sign In with Google

Adds Google OAuth alongside existing Apple and email/password auth. Follows the same patterns as Apple Sign-In: state cookie for CSRF, anonymousId migration, and user linking by email. Key differences: Google callback is a GET redirect (sameSite: lax) and uses a static client secret instead of a signed JWT.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/server/auth.test.ts

@@ -99,6 +99,7 @@ export async function createUser(anonymousId: string, email: string, passwordHas
 		email,
 		passwordHash,
 		appleId: null,
+		googleId: null,
 		firstName: firstName || null,
 		lastName: lastName || null,
 		isPrivate: false