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/HostDetailPage.tsx

@@ -64,11 +64,11 @@ 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}`
     }
   }

frontend/src/pages/HostsPage.tsx

@@ -6,8 +6,7 @@ import {
 } from '@mui/material'
 import { Add as AddIcon, Refresh as RefreshIcon, Delete as DeleteIcon } from '@mui/icons-material'
 import { useNavigate } from 'react-router-dom'
-import { apiClient } from '../api/client'
-import { hostsApi } from '../api/client'
+import { apiClient, hostsApi } from '../api/client'
 import type { Host, HostHealthStatus } from '../types'
 
 const statusColor = (s: HostHealthStatus) =>

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}`
     }
   }

frontend/src/pages/ReportsPage.tsx

@@ -126,7 +126,7 @@ export default function ReportsPage() {
       link.click()
       link.remove()
       window.URL.revokeObjectURL(url)
-    } catch (err: unknown) {
+    } catch (_err: unknown) {
       setError('Failed to generate report. Please try again.')
     } finally {
       setDownloading(false)