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/store/authStore.js

@@ -8,6 +8,7 @@ export const useAuthStore = create((set, get) => ({
   isAuthenticated: !!localStorage.getItem('auth_token'),
   isLoading: false,
   error: null,
+  demoMode: false,
 
   login: async (email, password) => {
     set({ isLoading: true, error: null })
@@ -129,4 +130,28 @@ export const useAuthStore = create((set, get) => ({
     await authApi.deleteAvatar()
     set(state => ({ user: { ...state.user, avatar_url: null } }))
   },
+
+  setDemoMode: (val) => set({ demoMode: val }),
+
+  demoLogin: async () => {
+    set({ isLoading: true, error: null })
+    try {
+      const data = await authApi.demoLogin()
+      localStorage.setItem('auth_token', data.token)
+      set({
+        user: data.user,
+        token: data.token,
+        isAuthenticated: true,
+        isLoading: false,
+        demoMode: true,
+        error: null,
+      })
+      connect(data.token)
+      return data
+    } catch (err) {
+      const error = err.response?.data?.error || 'Demo-Login fehlgeschlagen'
+      set({ isLoading: false, error })
+      throw new Error(error)
+    }
+  },
 }))