feat(theme): add appearance settings and theme provider integration

- Added theme selection in settings page for light, dark, and system modes.
- Integrated ThemeProvider into application layout.
- Updated dashboard layout to include theme toggle in the header.

frontend/src/app/(dashboard)/layout.tsx

@@ -8,6 +8,7 @@ import {
 	SidebarTrigger,
 } from "@/components/ui/sidebar";
 import { UserNavMobile } from "@/components/user-nav-mobile";
+import { ModeToggle } from "@/components/mode-toggle";
 
 export default function DashboardLayout({
 	children,
@@ -27,7 +28,10 @@ export default function DashboardLayout({
 							<div className="flex-1 flex justify-center">
 								<span className="font-bold text-primary text-lg">MemeGoat</span>
 							</div>
-							<UserNavMobile />
+							<div className="flex items-center gap-2">
+								<ModeToggle />
+								<UserNavMobile />
+							</div>
 						</header>
 						<main className="flex-1 overflow-y-auto bg-zinc-50 dark:bg-zinc-950">
 							{children}