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>

client/src/components/layout/Sidebar.tsx

@@ -4,6 +4,7 @@ import {
   MemoryStick,
   HardDrive,
   Wifi,
+  Bell,
   Settings,
   HelpCircle,
   Server,
@@ -24,6 +25,7 @@ const mainNav: NavItem[] = [
   { id: "memory", label: "Memory", icon: MemoryStick },
   { id: "storage", label: "Storage", icon: HardDrive },
   { id: "network", label: "Network", icon: Wifi },
+  { id: "notifications", label: "Notifications", icon: Bell },
 ]
 
 const bottomNav: NavItem[] = [