fix: add dashboard to navigation and set as default landing page (refs #2)

- Add Dashboard to desktop sidebar navigation (first item)
- Add /garage/dashboard route for desktop
- Change default redirect from /garage/vehicles to /garage/dashboard
- Change mobile default screen from Vehicles to Dashboard
- Create DashboardPage wrapper for desktop route

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/components/Layout.tsx

@@ -6,6 +6,7 @@ import React from 'react';
 import { useAuth0 } from '@auth0/auth0-react';
 import { Link, useLocation } from 'react-router-dom';
 import { Container, Paper, Typography, Box, IconButton, Avatar } from '@mui/material';
+import HomeRoundedIcon from '@mui/icons-material/HomeRounded';
 import DirectionsCarRoundedIcon from '@mui/icons-material/DirectionsCarRounded';
 import LocalGasStationRoundedIcon from '@mui/icons-material/LocalGasStationRounded';
 import BuildRoundedIcon from '@mui/icons-material/BuildRounded';
@@ -40,6 +41,7 @@ export const Layout: React.FC<LayoutProps> = ({ children, mobileMode = false })
   }, [mobileMode, setSidebarOpen]); // Removed sidebarOpen from dependencies
 
   const navigation = [
+    { name: 'Dashboard', href: '/garage/dashboard', icon: <HomeRoundedIcon sx={{ fontSize: 20 }} /> },
     { name: 'Vehicles', href: '/garage/vehicles', icon: <DirectionsCarRoundedIcon sx={{ fontSize: 20 }} /> },
     { name: 'Fuel Logs', href: '/garage/fuel-logs', icon: <LocalGasStationRoundedIcon sx={{ fontSize: 20 }} /> },
     { name: 'Maintenance', href: '/garage/maintenance', icon: <BuildRoundedIcon sx={{ fontSize: 20 }} /> },