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>

drizzle/0003_overjoyed_mindworm.sql

@@ -0,0 +1,3 @@
+ALTER TABLE `daily_completions` ADD `guesses` text;--> statement-breakpoint
+ALTER TABLE `user` ADD `google_id` text;--> statement-breakpoint
+CREATE UNIQUE INDEX `user_google_id_unique` ON `user` (`google_id`);