Fix all frontend ESLint errors

- Add eslint-plugin-react-hooks to package.json and config
- Fix duplicate imports in HostsPage.tsx
- Remove non-null assertion in main.tsx
- Fix != to !== in HostDetailPage.tsx and MaintenanceWindowsPage.tsx
- Fix unused variable in ReportsPage.tsx
- Change console.debug to console.warn in useJobWebSocket.ts

frontend/src/pages/MaintenanceWindowsPage.tsx

@@ -81,11 +81,11 @@ function scheduleDescription(w: MaintenanceWindow): string {
     case 'daily':
       return `Every day at ${time} for ${dur}`
     case 'weekly': {
-      const day = w.recurrence_day != null ? DAY_NAMES[w.recurrence_day] ?? `Day ${w.recurrence_day}` : '?'
+      const day = w.recurrence_day !== null ? DAY_NAMES[w.recurrence_day] ?? `Day ${w.recurrence_day}` : '?'
       return `Every ${day} at ${time} for ${dur}`
     }
     case 'monthly': {
-      const day = w.recurrence_day != null ? w.recurrence_day : '?'
+      const day = w.recurrence_day !== null ? w.recurrence_day : '?'
       return `Monthly on day ${day} at ${time} for ${dur}`
     }
   }