feat(admin): add system status panel with database, storage, and update checks

Add a comprehensive System Status page to the admin panel that provides
actionable insights for administrators to monitor their Umami instance health.

Features:
- Database connectivity check with connection status and database type
- Storage usage monitoring with total, used, and free space display
  - Visual progress bar with color-coded warnings (75% and 90% thresholds)
  - Human-readable byte formatting
  - Fallback support for different filesystem types
- Update notifications with version comparison using semver
  - Current version display
  - Latest version check via Umami API
  - Clear update availability indicators

Technical details:
- New API endpoint: /api/admin/status (admin-only access)
- New admin page: /admin/status
- Added to admin navigation menu with Activity icon
- Auto-refreshes every 60 seconds
- Proper error handling and graceful degradation
- Uses existing permission system (canViewUsers check)
- Follows existing UI patterns and component structure
- Added i18n labels: systemStatus, database, storage, updates

This addresses the need for administrators to have visibility into system
health and proactively identify potential issues before they impact users.

src/components/messages.ts

@@ -363,6 +363,10 @@ export const labels = defineMessages({
   support: { id: 'label.support', defaultMessage: 'Support' },
   documentation: { id: 'label.documentation', defaultMessage: 'Documentation' },
   switchAccount: { id: 'label.switch-account', defaultMessage: 'Switch account' },
+  systemStatus: { id: 'label.system-status', defaultMessage: 'System Status' },
+  database: { id: 'label.database', defaultMessage: 'Database' },
+  storage: { id: 'label.storage', defaultMessage: 'Storage' },
+  updates: { id: 'label.updates', defaultMessage: 'Updates' },
 });
 
 export const messages = defineMessages({