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/components/Layout.tsx

@@ -9,7 +9,6 @@ import {
   Plane,
   Users,
   Car,
-  Truck,
   Calendar,
   UserCog,
   LayoutDashboard,
@@ -71,8 +70,7 @@ export function Layout({ children }: LayoutProps) {
     { name: 'My Profile', href: '/profile', icon: UserCog, driverOnly: true },
     { name: 'War Room', href: '/command-center', icon: Radio, requireRead: 'ScheduleEvent' as const, coordinatorOnly: true },
     { name: 'VIPs', href: '/vips', icon: Users, requireRead: 'VIP' as const, coordinatorOnly: true },
-    { name: 'Drivers', href: '/drivers', icon: Car, requireRead: 'Driver' as const, coordinatorOnly: true },
-    { name: 'Vehicles', href: '/vehicles', icon: Truck, requireRead: 'Vehicle' as const, coordinatorOnly: true },
+    { name: 'Fleet', href: '/fleet', icon: Car, requireRead: 'Driver' as const, coordinatorOnly: true },
     { name: 'Activities', href: '/events', icon: Calendar, requireRead: 'ScheduleEvent' as const, coordinatorOnly: true },
     { name: 'Flights', href: '/flights', icon: Plane, requireRead: 'Flight' as const, coordinatorOnly: true },
   ];