fix: downgrade Next.js 16 to 15 for Vercel deployment compatibility

Next.js 16.1.6 is not yet fully supported by Vercel's build system,
causing 404 on all routes. Downgraded to Next.js 15.5.12 + React 18,
fixed ESLint config, TypeScript config, and lint errors.

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

package.json

@@ -13,19 +13,20 @@
     "@supabase/ssr": "^0.8.0",
     "@supabase/supabase-js": "^2.95.3",
     "clsx": "^2.1.1",
-    "next": "16.1.6",
-    "react": "19.2.3",
-    "react-dom": "19.2.3",
+    "next": "^15.5.12",
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1",
     "resend": "^6.9.1",
     "stripe": "^20.3.1"
   },
   "devDependencies": {
+    "@eslint/eslintrc": "^3.3.3",
     "@tailwindcss/postcss": "^4",
     "@types/node": "^20",
-    "@types/react": "^19",
-    "@types/react-dom": "^19",
+    "@types/react": "^18.3.28",
+    "@types/react-dom": "^18.3.7",
     "eslint": "^9",
-    "eslint-config-next": "16.1.6",
+    "eslint-config-next": "^15.5.12",
     "tailwindcss": "^4",
     "typescript": "^5"
   }