refactor(layout): unify mobile and desktop navigation and fix layout spacing

- Sync `MobileNav` items and icons with desktop `Sidebar` for consistency.
- Standardize menu icons using `lucide-react` across all navigation components.
- Add a spacer div in `main-layout.tsx` to ensure proper bottom spacing between content and the fixed footer.
- Remove redundant 'aria-hidden' attribute in `MobileNav` to fix accessibility warnings.
- Fix 'Logout' button layout in mobile menu for better responsiveness.

apps/web/src/components/layout/footer.tsx

@@ -1,6 +1,6 @@
 export function Footer() {
   return (
-    <footer className="h-22 shrink-0 border-t bg-[#ECF4E8] flex items-center justify-center">
+    <footer className="h-18 shrink-0 border-t bg-[#ECF4E8] flex items-center justify-center">
       <div className="mx-auto max-w-5xl px-6 py-6 text-sm text-muted-foreground text-center">
         © 2026 ZeaVis Edu - AI for Smart Education
       </div>

apps/web/src/components/layout/sidebar.tsx

@@ -114,7 +114,7 @@ export function Sidebar() {
 
       {/* Logout Button */}
       <div
-        className={`h-22 px-5 border-t border-white/10 shrink-0 flex items-center ${isSidebarOpen ? "" : "justify-center"}`}
+        className={`h-18 px-5 border-t border-white/10 shrink-0 flex items-center ${isSidebarOpen ? "" : "justify-center"}`}
       >
         <Link
           to="/logout"