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

server/src/routes/reservations.ts

@@ -101,6 +101,12 @@ router.post('/', authenticate, (req: Request, res: Response) => {
 
   res.status(201).json({ reservation });
   broadcast(tripId, 'reservation:created', { reservation }, req.headers['x-socket-id'] as string);
+
+  // Notify trip members about new booking
+  import('../services/notifications').then(({ notifyTripMembers }) => {
+    const tripInfo = db.prepare('SELECT title FROM trips WHERE id = ?').get(tripId) as { title: string } | undefined;
+    notifyTripMembers(Number(tripId), authReq.user.id, 'booking_change', { trip: tripInfo?.title || 'Untitled', actor: authReq.user.username, booking: title, type: type || 'booking' }).catch(() => {});
+  });
 });
 
 // Batch update day_plan_position for multiple reservations (must be before /:id)