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

tsconfig.json

@@ -11,7 +11,7 @@
     "moduleResolution": "bundler",
     "resolveJsonModule": true,
     "isolatedModules": true,
-    "jsx": "react-jsx",
+    "jsx": "preserve",
     "incremental": true,
     "plugins": [
       {
@@ -26,9 +26,7 @@
     "next-env.d.ts",
     "**/*.ts",
     "**/*.tsx",
-    ".next/types/**/*.ts",
-    ".next/dev/types/**/*.ts",
-    "**/*.mts"
+    ".next/types/**/*.ts"
   ],
   "exclude": ["node_modules"]
 }