Update semantic color tokens across components for improved consistency

- Refactor `text-*` and `bg-*` classes to use semantic color tokens such as `foreground`, `muted-foreground`, `card`, and `accent`.
- Adjust `Header`, `Footer`, and settings pages to align with the OKLCH-based design system.
- Ensure visual consistency and accessibility for both light and dark themes.

frontend/src/app/(authenticated)/settings/sessions/page.tsx

@@ -12,10 +12,10 @@ export const metadata: Metadata = {
 export default function SessionsPage() {
   return (
     <div>
-      <h2 className="text-2xl font-semibold text-gray-900 dark:text-white mb-4">
+      <h2 className="text-2xl font-semibold text-foreground mb-4">
         Active Sessions
       </h2>
-      <p className="text-gray-600 dark:text-gray-400">
+      <p className="text-muted-foreground">
         Manage your active sessions (Coming in Task 3.4)
       </p>
     </div>