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/ui/card.tsx

@@ -7,7 +7,7 @@ function Card({ className, ...props }: React.ComponentProps<"div">) {
     <div
       data-slot="card"
       className={cn(
-        "bg-card text-card-foreground border-border/40 flex flex-col border shadow-lg",
+        "bg-card text-card-foreground border-border/40 flex flex-col rounded-lg border shadow-lg",
         className,
       )}
       {...props}