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/theme/theme-switcher.tsx

@@ -0,0 +1,40 @@
+"use client";
+
+import * as React from "react";
+import { Moon, Sun } from "lucide-react";
+import { useTheme } from "next-themes";
+
+import { Button } from "~/components/ui/button";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from "~/components/ui/dropdown-menu";
+
+export function ThemeSwitcher() {
+  const { setTheme } = useTheme();
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="outline" size="icon">
+          <Sun className="h-[1.2rem] w-[1.2rem] rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
+          <Moon className="absolute h-[1.2rem] w-[1.2rem] rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
+          <span className="sr-only">Toggle theme</span>
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onClick={() => setTheme("light")}>
+          Light
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("dark")}>
+          Dark
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("system")}>
+          System
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}