feat: public read-only share links with permissions — closes #79

Share links:
- Generate a public link in the trip share modal
- Choose what to share: Map & Plan, Bookings, Packing, Budget, Chat
- Permissions enforced server-side
- Delete link to revoke access instantly

Shared trip page (/shared/:token):
- Read-only view with TREK logo, cover image, trip details
- Tabbed navigation with Lucide icons (responsive on mobile)
- Interactive map with auto-fit bounds per day
- Day plan, Bookings, Packing, Budget, Chat views
- Language picker, TREK branding footer

Technical:
- share_tokens DB table with per-field permissions
- Public GET /shared/:token endpoint (no auth)
- Two-column share modal (max-w-5xl)

client/src/api/client.ts

@@ -289,6 +289,13 @@ export const backupApi = {
   setAutoSettings: (settings: Record<string, unknown>) => apiClient.put('/backup/auto-settings', settings).then(r => r.data),
 }
 
+export const shareApi = {
+  getLink: (tripId: number | string) => apiClient.get(`/trips/${tripId}/share-link`).then(r => r.data),
+  createLink: (tripId: number | string, perms?: Record<string, boolean>) => apiClient.post(`/trips/${tripId}/share-link`, perms || {}).then(r => r.data),
+  deleteLink: (tripId: number | string) => apiClient.delete(`/trips/${tripId}/share-link`).then(r => r.data),
+  getSharedTrip: (token: string) => apiClient.get(`/shared/${token}`).then(r => r.data),
+}
+
 export const notificationsApi = {
   getPreferences: () => apiClient.get('/notifications/preferences').then(r => r.data),
   updatePreferences: (prefs: Record<string, boolean>) => apiClient.put('/notifications/preferences', prefs).then(r => r.data),