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/providers/theme-provider.tsx

@@ -0,0 +1,16 @@
+"use client";
+
+import * as React from "react";
+import { ThemeProvider as NextThemesProvider } from "next-themes";
+import { type ThemeProviderProps } from "next-themes/dist/types";
+
+export function ThemeProvider({ children, ...props }: ThemeProviderProps) {
+  return (
+    <NextThemesProvider
+      {...props}
+      themes={["light", "dark", "theme-sunset", "theme-forest"]}
+    >
+      {children}
+    </NextThemesProvider>
+  );
+}