Add Next.js frontend PWA

- Pages: login, register, tasks list, task detail
- Components: TaskCard, TaskForm, GroupSelector, StatusBadge, Header
- Tailwind CSS, dark/light mode, PWA manifest
- Running on :3001 via systemd

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/tasks/components/ThemeProvider.tsx

@@ -0,0 +1,54 @@
+"use client";
+
+import { createContext, useContext, useEffect, useState, ReactNode } from "react";
+
+type Theme = "light" | "dark";
+
+interface ThemeContextType {
+  theme: Theme;
+  toggleTheme: () => void;
+}
+
+const ThemeContext = createContext<ThemeContextType>({
+  theme: "light",
+  toggleTheme: () => {},
+});
+
+export function useTheme() {
+  return useContext(ThemeContext);
+}
+
+export default function ThemeProvider({ children }: { children: ReactNode }) {
+  const [theme, setTheme] = useState<Theme>("light");
+  const [mounted, setMounted] = useState(false);
+
+  useEffect(() => {
+    setMounted(true);
+    const stored = localStorage.getItem("taskteam_theme") as Theme | null;
+    if (stored) {
+      setTheme(stored);
+    } else if (window.matchMedia("(prefers-color-scheme: dark)").matches) {
+      setTheme("dark");
+    }
+  }, []);
+
+  useEffect(() => {
+    if (!mounted) return;
+    document.documentElement.classList.toggle("dark", theme === "dark");
+    localStorage.setItem("taskteam_theme", theme);
+  }, [theme, mounted]);
+
+  function toggleTheme() {
+    setTheme((prev) => (prev === "light" ? "dark" : "light"));
+  }
+
+  if (!mounted) {
+    return <>{children}</>;
+  }
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}