feat: email notifications, webhook support, ICS export — closes #110

Email Notifications:
- SMTP configuration in Admin > Settings (host, port, user, pass, from)
- App URL setting for email CTA links
- Webhook URL support (Discord, Slack, custom)
- Test email button with SMTP validation
- Beautiful HTML email template with TREK logo, slogan, red heart footer
- All notification texts translated in 8 languages (en/de/fr/es/nl/ru/zh/ar)
- Emails sent in each user's language preference

Notification Events:
- Trip invitation (member added)
- Booking created (new reservation)
- Vacay fusion invite
- Photos shared (Immich)
- Collab chat message
- Packing list category assignment

User Notification Preferences:
- Per-user toggle for each event type in Settings
- Addon-aware: Vacay/Collab/Photos toggles hidden when addon disabled
- Webhook opt-in per user

ICS Calendar Export:
- Download button next to PDF in day plan header
- Exports trip dates + all reservations with details
- Compatible with Google Calendar, Apple Calendar, Outlook

Technical:
- Nodemailer for SMTP
- notification_preferences DB table with per-event columns
- GET/PUT /auth/app-settings for admin config persistence
- POST /notifications/test-smtp for validation
- Dynamic imports for non-blocking notification sends

client/src/api/client.ts

@@ -289,4 +289,10 @@ export const backupApi = {
   setAutoSettings: (settings: Record<string, unknown>) => apiClient.put('/backup/auto-settings', settings).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),
+  testSmtp: (email?: string) => apiClient.post('/notifications/test-smtp', { email }).then(r => r.data),
+}
+
 export default apiClient