feat(frontend): add design tokens, dark mode hook, and i18n scaffold

Part of the Ombrora Convert redesign (Task 1/5).

frontend/src/hooks/useTheme.js

@@ -0,0 +1,27 @@
+import { useEffect, useState } from 'react';
+
+const STORAGE_KEY = 'theme';
+
+function getInitialTheme() {
+  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 useTheme() {
+  const [theme, setTheme] = useState(getInitialTheme);
+
+  useEffect(() => {
+    document.documentElement.setAttribute('data-theme', theme);
+  }, [theme]);
+
+  function toggleTheme() {
+    setTheme((current) => {
+      const next = current === 'dark' ? 'light' : 'dark';
+      localStorage.setItem(STORAGE_KEY, next);
+      return next;
+    });
+  }
+
+  return { theme, toggleTheme };
+}