Update Next.js to v15.5.6 and upgrade dependencies

Bump Next.js from 15.4.5 to 15.5.6 and update related dependencies.

Also upgrade other packages to latest compatible versions including: -
Radix UI components (all minor version updates) - Tiptap editor (3.0.7 →
3.11.0) - React and React DOM (19.1.1 → 19.2.0) - TanStack Query (5.84.0
→ 5.90.10) - TypeScript and ESLint ecosystem - Tailwind CSS (4.1.11 →
4.1.17) - Various other patch and minor updates

Additionally add theme support with next-themes and multiple color
schemes (light, dark, sunset, forest).

src/app/dashboard/_components/animated-stats-card.tsx

@@ -55,7 +55,7 @@ export function AnimatedStatsCard({
   void numericValue;
 
   return (
-    <Card className="stats-card">
+    <Card>
       <CardContent className="p-6">
         <div className="flex items-center justify-between space-y-0 pb-2">
           <div className="flex items-center space-x-2">