Add demo mode with hourly reset, example trips & demo banner

DEMO_MODE=true enables: auto-seeded admin + demo user, 3 example trips
(Tokyo, Barcelona, Wien), hourly reset of demo user data, one-click
demo login, visible banner with feature info. Zero behavior change
when DEMO_MODE is not set.

client/src/api/client.js

@@ -53,6 +53,7 @@ export const authApi = {
   updateAppSettings: (data) => apiClient.put('/auth/app-settings', data).then(r => r.data),
   validateKeys: () => apiClient.get('/auth/validate-keys').then(r => r.data),
   travelStats: () => apiClient.get('/auth/travel-stats').then(r => r.data),
+  demoLogin: () => apiClient.post('/auth/demo-login').then(r => r.data),
 }
 
 export const tripsApi = {