feat: add frontend auth pages, app layout, and Supabase integration (Phase 1E)

- Auth pages: login (password + magic link), register (with firm name), callback
- Supabase client setup: browser client, server client, middleware for session refresh
- App layout: sidebar (Dashboard, Akten, Fristen, Termine, AI Analyse, Einstellungen),
  header with user info and tenant switcher
- Shared: API client with auth headers, TypeScript types matching Go models,
  QueryClientProvider + Toaster providers
- Dependencies: @supabase/supabase-js, @supabase/ssr, @tanstack/react-query,
  lucide-react, date-fns, sonner

frontend/src/components/layout/Header.tsx

@@ -0,0 +1,44 @@
+"use client";
+
+import { createClient } from "@/lib/supabase/client";
+import { TenantSwitcher } from "./TenantSwitcher";
+import { LogOut } from "lucide-react";
+import { useRouter } from "next/navigation";
+import { useEffect, useState } from "react";
+
+export function Header() {
+  const [email, setEmail] = useState<string | null>(null);
+  const router = useRouter();
+  const supabase = createClient();
+
+  useEffect(() => {
+    supabase.auth.getUser().then(({ data: { user } }) => {
+      setEmail(user?.email ?? null);
+    });
+  }, [supabase.auth]);
+
+  async function handleLogout() {
+    await supabase.auth.signOut();
+    router.push("/login");
+    router.refresh();
+  }
+
+  return (
+    <header className="flex h-14 items-center justify-between border-b border-neutral-200 bg-white px-4">
+      <div />
+      <div className="flex items-center gap-3">
+        <TenantSwitcher />
+        {email && (
+          <span className="text-sm text-neutral-500">{email}</span>
+        )}
+        <button
+          onClick={handleLogout}
+          title="Abmelden"
+          className="rounded-md p-1.5 text-neutral-400 hover:bg-neutral-100 hover:text-neutral-600"
+        >
+          <LogOut className="h-4 w-4" />
+        </button>
+      </div>
+    </header>
+  );
+}