Add layout shell with header, footer, and theme toggle

frontend/src/components/layout/footer.tsx

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+import type { Globals, Menu } from '@/lib/types'
+
+type Props = {
+  globals: Globals
+  menu: Menu | undefined
+}
+
+export function Footer({ globals, menu }: Props) {
+  return (
+    <footer className="mt-8 bg-muted/50">
+      <div className="flex h-16 items-center justify-between gap-4 px-4">
+        <nav className="flex gap-1">
+          {menu?.items.map((item) => (
+            <a
+              key={item.label}
+              href={item.url ?? '#'}
+              className="px-2 py-1 text-sm font-medium text-muted-foreground transition-colors hover:text-foreground"
+            >
+              {item.label}
+            </a>
+          ))}
+        </nav>
+        <p className="text-sm text-muted-foreground">{globals.copyright}</p>
+      </div>
+    </footer>
+  )
+}

frontend/src/components/layout/header.tsx

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+import { Link } from '@tanstack/react-router'
+import type { Globals, Menu } from '@/lib/types'
+import { ThemeToggle } from './theme-toggle'
+
+type Props = {
+  globals: Globals
+  menu: Menu | undefined
+}
+
+export function Header({ globals, menu }: Props) {
+  return (
+    <header className="bg-muted/50">
+      <div className="flex h-16 items-center gap-4 px-4">
+        <Link to="/" className="text-base font-semibold hover:no-underline">
+          {globals.site_name}
+        </Link>
+        <nav className="flex gap-1 pl-2">
+          {menu?.items.map((item) => (
+            <a
+              key={item.label}
+              href={item.url ?? '#'}
+              className="px-2 py-1 text-sm font-medium text-muted-foreground transition-colors hover:text-foreground"
+            >
+              {item.label}
+            </a>
+          ))}
+        </nav>
+        <div className="ml-auto">
+          <ThemeToggle />
+        </div>
+      </div>
+    </header>
+  )
+}

frontend/src/components/layout/shell.tsx

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+import { useSuspenseQuery } from '@tanstack/react-query'
+import { globalsQuery, menusQuery } from '@/lib/queries'
+import { Header } from './header'
+import { Footer } from './footer'
+
+export function Shell({ children }: { children: React.ReactNode }) {
+  const { data: globals } = useSuspenseQuery(globalsQuery)
+  const { data: menus } = useSuspenseQuery(menusQuery)
+
+  const mainMenu = menus.find((m) => m.id === 'MAIN_MENU')
+  const footerMenu = menus.find((m) => m.id === 'FOOTER_MENU')
+
+  return (
+    <div className="mx-auto max-w-[90rem] px-4">
+      <Header globals={globals} menu={mainMenu} />
+      <main className="my-6">{children}</main>
+      <Footer globals={globals} menu={footerMenu} />
+    </div>
+  )
+}

frontend/src/components/layout/theme-toggle.tsx

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+import { Moon, Sun } from 'lucide-react'
+import { Button } from '@/components/ui/button'
+import { useTheme } from '@/components/theme-provider'
+
+export function ThemeToggle() {
+  const { theme, toggle } = useTheme()
+  return (
+    <Button variant="outline" size="icon" onClick={toggle} aria-label="Toggle theme">
+      {theme === 'light' ? <Moon /> : <Sun />}
+    </Button>
+  )
+}

frontend/src/components/theme-provider.tsx

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+import { createContext, useContext, useEffect, useState } from 'react'
+
+type Theme = 'light' | 'dark'
+
+type ThemeContextValue = {
+  theme: Theme
+  toggle: () => void
+  set: (t: Theme) => void
+}
+
+const ThemeContext = createContext<ThemeContextValue | null>(null)
+
+const STORAGE_KEY = 'theme'
+
+function readInitial(): Theme {
+  if (typeof window === 'undefined') return 'light'
+  const stored = localStorage.getItem(STORAGE_KEY)
+  if (stored === 'light' || stored === 'dark') return stored
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+}
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const [theme, setTheme] = useState<Theme>(readInitial)
+
+  useEffect(() => {
+    const root = document.documentElement
+    root.classList.remove('light', 'dark')
+    root.classList.add(theme)
+    localStorage.setItem(STORAGE_KEY, theme)
+  }, [theme])
+
+  return (
+    <ThemeContext.Provider
+      value={{
+        theme,
+        toggle: () => setTheme((t) => (t === 'light' ? 'dark' : 'light')),
+        set: setTheme,
+      }}
+    >
+      {children}
+    </ThemeContext.Provider>
+  )
+}
+
+export function useTheme() {
+  const ctx = useContext(ThemeContext)
+  if (!ctx) throw new Error('useTheme must be used inside ThemeProvider')
+  return ctx
+}