CUB-121: build React pages with real API integration

- HubPage: agent summary stats, cards, status badges, progress bars, refresh
- LogsPage: activity feed from tasks, status filter, loading skeleton
- ProjectsPage: project cards with status badges and agent counts
- SessionsPage: responsive table/card view with model/token info
- SettingsPage: dark mode toggle, gateway URL, refresh interval persist
- ThemeProvider with dark/light mode via CSS custom properties
- useLocalStorage hook for settings persistence
- Loading/error/empty states across all pages
- npm run build passes cleanly

frontend/src/hooks/useLocalStorage.ts

@@ -0,0 +1,29 @@
+import { useState, useEffect, useCallback } from 'react'
+
+export function useLocalStorage<T>(key: string, initialValue: T): [T, (value: T | ((prev: T) => T)) => void] {
+  const [storedValue, setStoredValue] = useState<T>(() => {
+    try {
+      const item = localStorage.getItem(key)
+      return item !== null ? (JSON.parse(item) as T) : initialValue
+    } catch {
+      return initialValue
+    }
+  })
+
+  useEffect(() => {
+    try {
+      localStorage.setItem(key, JSON.stringify(storedValue))
+    } catch {
+      // storage full or unavailable
+    }
+  }, [key, storedValue])
+
+  const setValue = useCallback((value: T | ((prev: T) => T)) => {
+    setStoredValue((prev) => {
+      const next = value instanceof Function ? value(prev) : value
+      return next
+    })
+  }, [])
+
+  return [storedValue, setValue]
+}