Add Supabase Auth with OAuth (Google, GitHub, X, Discord) and PostHog identification

- Add @supabase/supabase-js and @supabase/ssr dependencies
- Create browser and server Supabase client utilities
- Update middleware to refresh auth sessions on every request
- Add /auth/callback route for OAuth code exchange
- Add AuthListener component that calls posthog.identify/reset on auth state changes
- Create login page with Google, GitHub, X, and Discord OAuth + email/password
- Create protected /notifications page (redirects to login if unauthenticated)
- Add UserMenu (sign out button) to nav, visible only when logged in
- Add French translations for all new strings

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

Author: xrendan

package.json

@@ -34,6 +34,8 @@
     "@radix-ui/react-popover": "^1.1.15",
     "@radix-ui/react-select": "^2.2.6",
     "@radix-ui/react-slot": "^1.2.3",
+    "@supabase/ssr": "^0.8.0",
+    "@supabase/supabase-js": "^2.95.3",
     "@tailwindcss/forms": "^0.5.10",
     "@types/mdx": "^2.0.13",
     "class-variance-authority": "^0.7.1",