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/floating-action-bar.tsx

@@ -71,7 +71,7 @@ export function FloatingActionBar({
           <CardContent className="flex items-center justify-between p-4">
             {/* Left content */}
             {leftContent && (
-              <div className="animate-fade-in flex flex-1 items-center gap-3">
+              <div className="text-card-foreground animate-fade-in flex flex-1 items-center gap-3">
                 {leftContent}
               </div>
             )}