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/app/(auth)/register/page.tsx

@@ -0,0 +1,151 @@
+"use client";
+
+import { createClient } from "@/lib/supabase/client";
+import { api } from "@/lib/api";
+import { useRouter } from "next/navigation";
+import { useState } from "react";
+
+export default function RegisterPage() {
+  const [email, setEmail] = useState("");
+  const [password, setPassword] = useState("");
+  const [firmName, setFirmName] = useState("");
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const router = useRouter();
+  const supabase = createClient();
+
+  async function handleRegister(e: React.FormEvent) {
+    e.preventDefault();
+    setLoading(true);
+    setError(null);
+
+    // 1. Create auth user
+    const { data, error: authError } = await supabase.auth.signUp({
+      email,
+      password,
+      options: {
+        emailRedirectTo: `${window.location.origin}/callback`,
+      },
+    });
+
+    if (authError) {
+      setError(authError.message);
+      setLoading(false);
+      return;
+    }
+
+    // 2. Create tenant via backend (the backend adds the user as owner)
+    if (data.session) {
+      try {
+        await api.post("/tenants", { name: firmName });
+      } catch (err: unknown) {
+        const apiErr = err as { error?: string };
+        setError(apiErr.error || "Kanzlei konnte nicht erstellt werden");
+        setLoading(false);
+        return;
+      }
+
+      router.push("/");
+      router.refresh();
+    } else {
+      // Email confirmation required
+      router.push("/login");
+    }
+
+    setLoading(false);
+  }
+
+  return (
+    <div className="flex min-h-screen items-center justify-center bg-neutral-50">
+      <div className="w-full max-w-sm space-y-6 rounded-lg border border-neutral-200 bg-white p-8">
+        <div className="text-center">
+          <h1 className="text-lg font-semibold text-neutral-900">
+            KanzlAI
+          </h1>
+          <p className="mt-1 text-sm text-neutral-500">
+            Erstellen Sie Ihr Konto
+          </p>
+        </div>
+
+        <form onSubmit={handleRegister} className="space-y-4">
+          <div>
+            <label
+              htmlFor="firm"
+              className="block text-sm font-medium text-neutral-700"
+            >
+              Kanzleiname
+            </label>
+            <input
+              id="firm"
+              type="text"
+              value={firmName}
+              onChange={(e) => setFirmName(e.target.value)}
+              required
+              className="mt-1 block w-full rounded-md border border-neutral-300 px-3 py-2 text-sm placeholder-neutral-400 focus:border-neutral-900 focus:outline-none focus:ring-1 focus:ring-neutral-900"
+              placeholder="Muster & Partner Rechtsanwaelte"
+            />
+          </div>
+
+          <div>
+            <label
+              htmlFor="email"
+              className="block text-sm font-medium text-neutral-700"
+            >
+              E-Mail
+            </label>
+            <input
+              id="email"
+              type="email"
+              value={email}
+              onChange={(e) => setEmail(e.target.value)}
+              required
+              className="mt-1 block w-full rounded-md border border-neutral-300 px-3 py-2 text-sm placeholder-neutral-400 focus:border-neutral-900 focus:outline-none focus:ring-1 focus:ring-neutral-900"
+              placeholder="anwalt@kanzlei.de"
+            />
+          </div>
+
+          <div>
+            <label
+              htmlFor="password"
+              className="block text-sm font-medium text-neutral-700"
+            >
+              Passwort
+            </label>
+            <input
+              id="password"
+              type="password"
+              value={password}
+              onChange={(e) => setPassword(e.target.value)}
+              required
+              minLength={8}
+              className="mt-1 block w-full rounded-md border border-neutral-300 px-3 py-2 text-sm placeholder-neutral-400 focus:border-neutral-900 focus:outline-none focus:ring-1 focus:ring-neutral-900"
+            />
+            <p className="mt-1 text-xs text-neutral-400">Mindestens 8 Zeichen</p>
+          </div>
+
+          {error && (
+            <p className="text-sm text-red-600">{error}</p>
+          )}
+
+          <button
+            type="submit"
+            disabled={loading}
+            className="w-full rounded-md bg-neutral-900 px-4 py-2 text-sm font-medium text-white hover:bg-neutral-800 focus:outline-none focus:ring-2 focus:ring-neutral-900 focus:ring-offset-2 disabled:opacity-50"
+          >
+            {loading ? "..." : "Konto erstellen"}
+          </button>
+        </form>
+
+        <p className="text-center text-sm text-neutral-500">
+          Bereits registriert?{" "}
+          <a
+            href="/login"
+            className="font-medium text-neutral-900 hover:underline"
+          >
+            Anmelden
+          </a>
+        </p>
+      </div>
+    </div>
+  );
+}