feat(auth): add Supabase auth with Google, GitHub, and email OTP login

Introduce user registration/login gated behind optional NEXT_PUBLIC_SUPABASE_*
env vars (leave blank to disable — app behaves exactly as before). Adds
proxy.ts for automatic cookie session refresh, requireUser() API route
guards on all 7 compute-consuming routes, AuthModal (Google/GitHub OAuth +
6-digit email OTP), UserChip header component, and login_success analytics
event. Identity is fully decoupled from Session/engine — no type changes.

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

lib/analytics.ts

@@ -54,6 +54,7 @@ type AnalyticsEventData = {
   fullscreen_toggle: { on: boolean };
   play_heartbeat: never;
   gallery_export: { scene_count: number; audio_count: number };
+  login_success: { provider: "google" | "github" | "email" };
 };
 
 export type AnalyticsEvent = keyof AnalyticsEventData;