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/components/layout/page-header.tsx

@@ -52,7 +52,7 @@ export function PageHeader({
           )}
         </div>
         {children && (
-          <div className="animate-slide-in-right animate-delay-200 flex flex-shrink-0 gap-2 sm:gap-3 [&>*]:h-8 [&>*]:px-2 [&>*]:text-sm sm:[&>*]:h-10 sm:[&>*]:px-4 sm:[&>*]:text-base [&>*>span]:hidden sm:[&>*>span]:inline [&>*>svg]:mr-0 sm:[&>*>svg]:mr-2">
+          <div className="animate-slide-in-right animate-delay-200 flex flex-shrink-0 gap-2 sm:gap-3">
             {children}
           </div>
         )}