feat: Implement an Apple-inspired design system with dark/light modes and introduce the initial dashboard layout including a heatmap grid component.

src/app/(dashboard)/layout.tsx

@@ -7,7 +7,7 @@ export default async function DashboardLayout({ children }: { children: React.Re
             <Sidebar />
             <main className="flex-1 flex flex-col h-full min-w-0 overflow-hidden">
                 <Header />
-                <div className="flex-1 overflow-auto p-4 md:p-8">
+                <div className="flex-1 overflow-hidden flex flex-col p-4 md:p-8">
                     {children}
                 </div>
             </main>

src/app/globals.css

@@ -192,8 +192,8 @@
 
 /* ── Refined scrollbar ── */
 .overflow-auto::-webkit-scrollbar {
-  width: 5px;
-  height: 5px;
+  width: 8px;
+  height: 8px;
 }
 
 .overflow-auto::-webkit-scrollbar-track {