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/db/schema.ts

@@ -7,6 +7,7 @@ export const user = sqliteTable('user', {
 	email: text('email').unique(),
 	passwordHash: text('password_hash'),
 	appleId: text('apple_id').unique(),
+	googleId: text('google_id').unique(),
 	isPrivate: integer('is_private', { mode: 'boolean' }).default(false)
 });