Add public product landing page at /

Serve a marketing landing page at the site root via ProductLandingController
instead of redirecting straight to login, with config/product_landing.php and
product views.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Http/Controllers/ProductLandingController.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use Illuminate\Http\RedirectResponse;
+use Illuminate\Http\Request;
+use Illuminate\View\View;
+
+class ProductLandingController extends Controller
+{
+    public function show(Request $request): View|RedirectResponse
+    {
+        $landing = config('product_landing', []);
+        $variant = (string) ($landing['landing_variant'] ?? 'default');
+        $dashboardRoute = (string) ($landing['dashboard_route'] ?? 'login');
+
+        if ($variant === 'webmail') {
+            if ($request->session()->has('mb.email')) {
+                return redirect()->route($dashboardRoute);
+            }
+
+            return view('product.landing');
+        }
+
+        if (auth()->check()) {
+            return redirect()->route($dashboardRoute);
+        }
+
+        return view('product.landing');
+    }
+}