chore: use display name instead of email in header greeting (refs #177)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -263,7 +263,7 @@ export const Layout: React.FC<LayoutProps> = ({ children, mobileMode = false })
             <Box sx={{ display: 'flex', alignItems: 'center', gap: 2 }}>
               <NotificationBell />
               <Typography variant="body2" color="text.secondary">
-                Welcome back, {user?.name || user?.email}
+                Welcome back, {user?.given_name || user?.name?.split(' ')[0] || user?.nickname || user?.email}
               </Typography>
             </Box>
           </Box>