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).

tailwind.config.ts

@@ -2,7 +2,7 @@ import type { Config } from "tailwindcss";
 import animate from "tailwindcss-animate";
 
 export default {
-  darkMode: "media",
+  darkMode: "class",
   content: [
     "./src/pages/**/*.{js,ts,jsx,tsx,mdx}",
     "./src/components/**/*.{js,ts,jsx,tsx,mdx}",
@@ -14,9 +14,9 @@ export default {
         xs: "475px",
       },
       fontFamily: {
-        sans: ["var(--font-geist-mono)", "monospace"],
+        sans: ["var(--font-geist-sans)", "sans-serif"],
         mono: ["var(--font-geist-mono)", "monospace"],
-        serif: ["var(--font-geist-mono)", "monospace"],
+        serif: ["var(--font-serif)", "serif"],
       },
       colors: {
         navbar: "var(--navbar)",
@@ -37,6 +37,11 @@ export default {
         "accordion-down": "accordion-down 0.2s ease-out",
         "accordion-up": "accordion-up 0.2s ease-out",
       },
+      borderRadius: {
+        lg: `var(--radius)`,
+        md: `calc(var(--radius) - 2px)`,
+        sm: `calc(var(--radius) - 4px)`,
+      },
     },
   },
   plugins: [animate],