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/package.json

@@ -17,9 +17,10 @@
     "jsonwebtoken": "^9.0.2",
     "multer": "^2.1.1",
     "node-cron": "^4.2.1",
+    "node-fetch": "^2.7.0",
+    "nodemailer": "^8.0.4",
     "otplib": "^12.0.1",
     "qrcode": "^1.5.4",
-    "node-fetch": "^2.7.0",
     "tsx": "^4.21.0",
     "typescript": "^6.0.2",
     "unzipper": "^0.12.3",
@@ -36,6 +37,7 @@
     "@types/multer": "^2.1.0",
     "@types/node": "^25.5.0",
     "@types/node-cron": "^3.0.11",
+    "@types/nodemailer": "^7.0.11",
     "@types/qrcode": "^1.5.5",
     "@types/unzipper": "^0.10.11",
     "@types/uuid": "^10.0.0",