feat: Add admin vehicle management and profile vehicles display (refs #11)

- Add GET /api/admin/stats endpoint for Total Vehicles widget
- Add GET /api/admin/users/:auth0Sub/vehicles endpoint for user vehicle list
- Update AdminUsersPage with Total Vehicles stat and expandable vehicle rows
- Add My Vehicles section to SettingsPage (desktop) and MobileSettingsScreen
- Update AdminUsersMobileScreen with stats header and vehicle expansion
- Add defense-in-depth admin checks and error handling
- Update admin README documentation

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

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

frontend/src/pages/SettingsPage.tsx

@@ -9,6 +9,7 @@ import { useUnits } from '../core/units/UnitsContext';
 import { useAdminAccess } from '../core/auth/useAdminAccess';
 import { useProfile, useUpdateProfile } from '../features/settings/hooks/useProfile';
 import { useExportUserData } from '../features/settings/hooks/useExportUserData';
+import { useVehicles } from '../features/vehicles/hooks/useVehicles';
 import { useTheme } from '../shared-minimal/theme/ThemeContext';
 import { DeleteAccountDialog } from '../features/settings/components/DeleteAccountDialog';
 import { PendingDeletionBanner } from '../features/settings/components/PendingDeletionBanner';
@@ -36,6 +37,7 @@ import PaletteIcon from '@mui/icons-material/Palette';
 import SecurityIcon from '@mui/icons-material/Security';
 import StorageIcon from '@mui/icons-material/Storage';
 import AdminPanelSettingsIcon from '@mui/icons-material/AdminPanelSettings';
+import DirectionsCarIcon from '@mui/icons-material/DirectionsCar';
 import EditIcon from '@mui/icons-material/Edit';
 import SaveIcon from '@mui/icons-material/Save';
 import CancelIcon from '@mui/icons-material/Cancel';
@@ -53,6 +55,9 @@ export const SettingsPage: React.FC = () => {
   // Profile state
   const { data: profile, isLoading: profileLoading } = useProfile();
   const updateProfileMutation = useUpdateProfile();
+
+  // Vehicles state (for My Vehicles section)
+  const { data: vehicles, isLoading: vehiclesLoading } = useVehicles();
   const [isEditingProfile, setIsEditingProfile] = useState(false);
   const [editedDisplayName, setEditedDisplayName] = useState('');
   const [editedNotificationEmail, setEditedNotificationEmail] = useState('');
@@ -277,6 +282,62 @@ export const SettingsPage: React.FC = () => {
           )}
         </Card>
 
+        {/* My Vehicles Section */}
+        <Card>
+          <Box sx={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center', mb: 3 }}>
+            <Box sx={{ display: 'flex', alignItems: 'center', gap: 1 }}>
+              <DirectionsCarIcon color="primary" />
+              <Typography variant="h6" sx={{ fontWeight: 600 }}>
+                My Vehicles
+              </Typography>
+              {!vehiclesLoading && vehicles && (
+                <Typography variant="body2" color="text.secondary">
+                  ({vehicles.length})
+                </Typography>
+              )}
+            </Box>
+            <MuiButton
+              variant="contained"
+              size="small"
+              onClick={() => navigate('/garage')}
+              sx={{
+                backgroundColor: 'primary.main',
+                color: 'primary.contrastText',
+                '&:hover': {
+                  backgroundColor: 'primary.dark'
+                }
+              }}
+            >
+              Manage
+            </MuiButton>
+          </Box>
+
+          {vehiclesLoading ? (
+            <Box sx={{ display: 'flex', justifyContent: 'center', py: 3 }}>
+              <CircularProgress size={24} />
+            </Box>
+          ) : !vehicles?.length ? (
+            <Typography variant="body2" color="text.secondary" sx={{ py: 2 }}>
+              No vehicles registered. Add your first vehicle to get started.
+            </Typography>
+          ) : (
+            <List disablePadding>
+              {vehicles.map((vehicle, index) => (
+                <React.Fragment key={vehicle.id}>
+                  {index > 0 && <Divider />}
+                  <ListItem sx={{ py: 1.5 }}>
+                    <ListItemText
+                      primary={`${vehicle.year} ${vehicle.make} ${vehicle.model}`}
+                      secondary={vehicle.nickname || undefined}
+                      primaryTypographyProps={{ fontWeight: 500 }}
+                    />
+                  </ListItem>
+                </React.Fragment>
+              ))}
+            </List>
+          )}
+        </Card>
+
         {/* Notifications Section */}
         <Card>
           <Typography variant="h6" sx={{ fontWeight: 600, mb: 3 }}>