feat: add Settings (user management) and Notifications pages

Backend:
- User management API: GET/POST/PUT/DELETE /api/users
  Supports two roles: developer, server-admin
  Each user has notifyOnError toggle for email alerts
- Notification API: GET /api/notifications, mark read, unread count
- Email service: sends alerts via SMTP (mail.bshtech.net/noreply@bshtech.net)
  to all users with notifyOnError=true on error/warning events
- Health monitor: checks CPU, memory, disk, Docker containers every 60s
  Creates notifications + sends email on threshold breaches
  Detects container down, high CPU (>90%), memory (>90%), disk (>85%)
  Sends recovery notifications when issues resolve
- File-based JSON storage for users and notifications (data/ directory)
- Added nodemailer dependency

Frontend:
- Settings page: user list with role badges, email alert toggles,
  add/remove user forms. Roles: Developer (blue), Server Admin (amber)
- Notifications page: real-time alert feed with type icons (error/warning/
  info/recovery), read/unread state, email sent indicator, time ago display
- Added Notifications to sidebar navigation
- Wired both pages into App.tsx router

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/package.json

@@ -12,12 +12,14 @@
     "dotenv": "^16.4.0",
     "express": "^4.21.0",
     "jsonwebtoken": "^9.0.3",
+    "nodemailer": "^6.9.0",
     "ssh2": "^1.16.0"
   },
   "devDependencies": {
     "@types/cors": "^2.8.17",
     "@types/express": "^5.0.0",
     "@types/jsonwebtoken": "^9.0.10",
+    "@types/nodemailer": "^6.4.0",
     "@types/ssh2": "^1.15.0",
     "tsx": "^4.19.0",
     "typescript": "^5.8.0"