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/App.tsx

@@ -11,6 +11,7 @@ import AdminPage from './pages/AdminPage'
 import SettingsPage from './pages/SettingsPage'
 import VacayPage from './pages/VacayPage'
 import AtlasPage from './pages/AtlasPage'
+import SharedTripPage from './pages/SharedTripPage'
 import { ToastContainer } from './components/shared/Toast'
 import { TranslationProvider, useTranslation } from './i18n'
 import DemoBanner from './components/Layout/DemoBanner'
@@ -128,6 +129,7 @@ export default function App() {
       <Routes>
         <Route path="/" element={<RootRedirect />} />
         <Route path="/login" element={<LoginPage />} />
+        <Route path="/shared/:token" element={<SharedTripPage />} />
         <Route path="/register" element={<LoginPage />} />
         <Route
           path="/dashboard"