feat: add theme system with dark/light/system support and persistence

apps/web/src/hooks/use-theme.ts

@@ -0,0 +1,22 @@
+import { useEffect } from "react";
+import { useThemeStore } from "../stores/theme-store";
+
+export function useTheme() {
+  const { theme, resolvedTheme, setTheme } = useThemeStore();
+
+  useEffect(() => {
+    const resolved =
+      theme === "system"
+        ? window.matchMedia("(prefers-color-scheme: dark)").matches
+          ? "dark"
+          : "light"
+        : theme;
+    document.documentElement.classList.toggle("dark", resolved === "dark");
+  }, [theme]);
+
+  const toggleTheme = () => {
+    setTheme(resolvedTheme === "dark" ? "light" : "dark");
+  };
+
+  return { theme, resolvedTheme, setTheme, toggleTheme };
+}