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

app/globals.css

@@ -1,5 +1,5 @@
-@import "tailwindcss";
 @import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700;800;900&display=swap');
+@import "tailwindcss";
 
 @theme inline {
   --color-primary: #6D5EF6;