feat(monitor): proportion-based overall status + reusable auto-refresh

- Change overall status logic: >50% failed = UNAVAILABLE, any failed
  or degraded = DEGRADED, all ok = OPERATIONAL
- Extract useAutoRefresh composable with localStorage persistence
- Create AutoRefreshButton dropdown component (reusable)
- Integrate auto-refresh into channel status page via MonitorHero

frontend/src/i18n/locales/en.ts

@@ -308,6 +308,12 @@ export default {
     saving: 'Saving...',
     selectedCount: '({count} selected)',
     refresh: 'Refresh',
+    autoRefresh: {
+      title: 'Auto Refresh',
+      enable: 'Enable auto refresh',
+      countdown: 'Auto refresh: {seconds}s',
+      seconds: '{n} seconds',
+    },
     view: 'View',
     settings: 'Settings',
     chooseFile: 'Choose File',