fix(alerts): harden with security fixes, design compliance, and test coverage

[AnsoCode]

Summary

• Add authMiddleware to all alert endpoints (GET/POST/DELETE /api/alerts) that were previously unprotected
• Validate notification test dispatch inputs (type must be discord/slack/webhook, URL must be HTTPS)
• Fix restart_count metric that was hardcoded to 0; now fetches via Docker inspect API
• Correct network metric unit labels from MB/s to MB (cumulative, not rate)
• Replace any types on Docker stats calculation methods with proper DockerContainerStats interface
• Add 10-second timeout (AbortSignal.timeout) to all webhook fetch calls
• Track and persist dispatch errors to notification history records
• Add update availability notifications: Sencho version updates (6-hour check in MonitorService) and stack image updates (state transition detection in ImageUpdateService)
• Extract shared version fetch logic from index.ts into utils/version-check.ts
• Frontend: migrate Select to Combobox, add ScrollArea, add delete confirmation AlertDialog, fix icon strokeWidth
• Add diagnostic logging gated behind developer_mode for MonitorService and NotificationService
• 24 new alert API integration tests + restart_count and version check unit tests (688 total passing)
• Update docs with HTTPS requirement, update notifications section, and troubleshooting guide

Test plan

• Backend TypeScript compiles clean (tsc --noEmit)
• Frontend TypeScript compiles clean (tsc --noEmit)
• All 688 backend tests pass (npm test)
• Manual: verify alert endpoints reject unauthenticated requests
• Manual: verify Combobox dropdowns work in alert sheet
• Manual: verify delete confirmation dialog appears before removing a rule
• Manual: verify ScrollArea scrolls correctly in the alert sheet