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/useTheme.tsx

@@ -0,0 +1,50 @@
+import { createContext, useContext, useState, useEffect, useCallback } from 'react'
+
+type Theme = 'dark' | 'light'
+
+interface ThemeContextValue {
+  theme: Theme
+  toggleTheme: () => void
+  isDark: boolean
+}
+
+const ThemeContext = createContext<ThemeContextValue | null>(null)
+
+function getInitialTheme(): Theme {
+  if (typeof window === 'undefined') return 'dark'
+  const stored = localStorage.getItem('cc-theme')
+  if (stored === 'light' || stored === 'dark') return stored
+  return window.matchMedia('(prefers-color-scheme: light)').matches ? 'light' : 'dark'
+}
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const [theme, setTheme] = useState<Theme>(getInitialTheme)
+
+  useEffect(() => {
+    const root = document.documentElement
+    if (theme === 'dark') {
+      root.classList.add('dark')
+      root.classList.remove('light')
+    } else {
+      root.classList.add('light')
+      root.classList.remove('dark')
+    }
+    localStorage.setItem('cc-theme', theme)
+  }, [theme])
+
+  const toggleTheme = useCallback(() => {
+    setTheme((prev) => (prev === 'dark' ? 'light' : 'dark'))
+  }, [])
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme, isDark: theme === 'dark' }}>
+      {children}
+    </ThemeContext.Provider>
+  )
+}
+
+export function useTheme() {
+  const ctx = useContext(ThemeContext)
+  if (!ctx) throw new Error('useTheme must be used within ThemeProvider')
+  return ctx
+}