fix: remove @supabase/ssr entirely to fix MIDDLEWARE_INVOCATION_FAILED

@supabase/ssr uses __dirname internally which crashes in Vercel's Edge
runtime, causing MIDDLEWARE_INVOCATION_FAILED even without a middleware
file. Replaced with @supabase/supabase-js directly:
- Server client: manual cookie-based session restoration
- Browser client: direct createClient from supabase-js
- Admin client: already using supabase-js

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

package.json

@@ -10,7 +10,6 @@
   },
   "dependencies": {
     "@stripe/stripe-js": "^8.7.0",
-    "@supabase/ssr": "^0.8.0",
     "@supabase/supabase-js": "^2.95.3",
     "clsx": "^2.1.1",
     "next": "^15.5.12",