fix(web): remove the mobile bottom navigation bar

The fixed bottom tab bar (MobileTabBar) was rendered app-wide on
screens <768px, duplicating navigation that already exists in the
header/hamburger menus.

- unmount <MobileTabBar /> in App.tsx and drop its import
- delete components/layout/MobileTabBar.tsx (no remaining references)
- drop the max-md:pb-24 clearance in PageLayout (Header.tsx) that only
  existed to keep page content above the removed fixed bar

Desktop and header-menu navigation are unaffected; mobile pages keep a
full-height viewport without dead bottom padding.

frontend/src/App.tsx

@@ -1,7 +1,6 @@
 import { lazy, Suspense } from "react";
 import { Navigate, Route, Routes } from "react-router-dom";
 import { useAuthStore } from "./store/auth";
-import { MobileTabBar } from "./components/layout/MobileTabBar";
 import { ErrorBoundary } from "./components/ErrorBoundary";
 
 // هر صفحه با React.lazy لود می‌شود (code splitting)
@@ -197,7 +196,6 @@ export default function App() {
         <Route path="*" element={<Navigate to="/" replace />} />
         </Routes>
       </Suspense>
-      <MobileTabBar />
     </ErrorBoundary>
   );
 }