feat: layout components (Sidebar, Header, MobileNav, TabBar, AppLayout) with routing

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/hooks/useTheme.ts

@@ -0,0 +1,23 @@
+import { useState, useEffect } from 'react'
+
+type Theme = 'dark' | 'light' | 'system'
+
+export function useTheme() {
+  const [theme, setTheme] = useState<Theme>(() => {
+    return (localStorage.getItem('hive-theme') as Theme) || 'system'
+  })
+
+  useEffect(() => {
+    const root = document.documentElement
+    if (theme === 'system') {
+      const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches
+      root.setAttribute('data-theme', prefersDark ? 'dark' : 'light')
+    } else {
+      root.setAttribute('data-theme', theme)
+    }
+    localStorage.setItem('hive-theme', theme)
+  }, [theme])
+
+  const toggle = () => setTheme(t => (t === 'dark' ? 'light' : 'dark'))
+  return { theme, setTheme, toggle }
+}