refactor(nav): group minigame links into Exercises submenu

Collapse the four training minigame entries (Flight Simulator,
Minefield Clearance, Radio Traffic, Qualification Course) into a
single DropdownMenuSub under the user menu.

src/components/frontend/blocks/NavUser.tsx

@@ -7,6 +7,7 @@ import {
   ChevronsUpDown,
   Crosshair,
   FileTextIcon,
+  Gamepad2,
   LogOut,
   Radio,
   Users2,
@@ -23,6 +24,9 @@ import {
   DropdownMenuItem,
   DropdownMenuLabel,
   DropdownMenuSeparator,
+  DropdownMenuSub,
+  DropdownMenuSubContent,
+  DropdownMenuSubTrigger,
   DropdownMenuTrigger,
 } from "@/components/ui/dropdown-menu";
 import {
@@ -132,6 +136,12 @@ export function NavUser({
                   Service Record
                 </Link>
               </DropdownMenuItem>
+              <DropdownMenuSub>
+                <DropdownMenuSubTrigger>
+                  <Gamepad2 />
+                  Exercises
+                </DropdownMenuSubTrigger>
+                <DropdownMenuSubContent>
                   <DropdownMenuItem asChild>
                     <Link href="/flappy">
                       <Bird />
@@ -156,6 +166,8 @@ export function NavUser({
                       Qualification Course
                     </Link>
                   </DropdownMenuItem>
+                </DropdownMenuSubContent>
+              </DropdownMenuSub>
             </DropdownMenuGroup>
             <DropdownMenuSeparator />
             <DropdownMenuGroup>