feat: consolidate Drivers and Vehicles into tabbed Fleet page

Replaces separate /drivers and /vehicles routes with a single /fleet
page using tabs. Old routes redirect for backward compatibility.
Navigation sidebar now shows one "Fleet" item instead of two.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/CommandCenter.tsx

@@ -378,7 +378,7 @@ export function CommandCenter() {
     alerts.push({
       type: 'warning',
       message: 'No vehicles available - all vehicles in use',
-      link: `/vehicles`,
+      link: `/fleet?tab=vehicles`,
     });
   }
 
@@ -902,7 +902,7 @@ export function CommandCenter() {
               Events
             </Link>
             <Link
-              to="/drivers"
+              to="/fleet?tab=drivers"
               className="flex items-center justify-center gap-1 px-2 py-2 bg-purple-100 dark:bg-purple-900/30 text-purple-700 dark:text-purple-300 rounded text-xs font-medium hover:bg-purple-200 dark:hover:bg-purple-900/50"
             >
               <Users className="h-3 w-3" />
@@ -916,7 +916,7 @@ export function CommandCenter() {
               VIPs
             </Link>
             <Link
-              to="/vehicles"
+              to="/fleet?tab=vehicles"
               className="flex items-center justify-center gap-1 px-2 py-2 bg-orange-100 dark:bg-orange-900/30 text-orange-700 dark:text-orange-300 rounded text-xs font-medium hover:bg-orange-200 dark:hover:bg-orange-900/50"
             >
               <Car className="h-3 w-3" />