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

eslint.config.mjs

@@ -1,18 +1,16 @@
-import { defineConfig, globalIgnores } from "eslint/config";
-import nextVitals from "eslint-config-next/core-web-vitals";
-import nextTs from "eslint-config-next/typescript";
+import { dirname } from "path";
+import { fileURLToPath } from "url";
+import { FlatCompat } from "@eslint/eslintrc";
 
-const eslintConfig = defineConfig([
-  ...nextVitals,
-  ...nextTs,
-  // Override default ignores of eslint-config-next.
-  globalIgnores([
-    // Default ignores of eslint-config-next:
-    ".next/**",
-    "out/**",
-    "build/**",
-    "next-env.d.ts",
-  ]),
-]);
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+
+const compat = new FlatCompat({
+  baseDirectory: __dirname,
+});
+
+const eslintConfig = [
+  ...compat.extends("next/core-web-vitals", "next/typescript"),
+];
 
 export default eslintConfig;