Add M3 error pages and the Markdown mail theme

Error pages for 401 to 503 in an error-view root appended to view.paths,
with an inline fallback when the Vite build is missing, and a mail theme
rendered from the application's scheme JSON, with opt-in header and
message components. Completes Phase 9.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9NnLxnPp8vaaurb3Z1MFy

src/Http/Controllers/ShowcasePageController.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace NoNameWeb\LivewireMaterial\Http\Controllers;
+
+use Illuminate\Http\Response;
+use Illuminate\Mail\Markdown;
+use NoNameWeb\LivewireMaterial\LivewireMaterialServiceProvider;
+
+/**
+ * Previews of the pages the showcase cannot draw inline: an error page, raised for real so the
+ * exception handler picks the view exactly as it would in the application (an app's own
+ * `resources/views/errors` included), and a sample Markdown mail in the package theme.
+ */
+class ShowcasePageController
+{
+    public function error(string $code): never
+    {
+        abort((int) $code);
+    }
+
+    /**
+     * The sample mail, with the package's header and message after the application's own mail
+     * components — what an application gets with `livewire-material.mail.components` on.
+     */
+    public function mail(): Response
+    {
+        $markdown = new Markdown(view(), [
+            'theme' => 'livewire-material::mail.theme',
+            'paths' => array_values(array_unique([
+                ...(array) config('mail.markdown.paths', []),
+                LivewireMaterialServiceProvider::mailComponentPath(),
+            ])),
+        ]);
+
+        return response($markdown->render('livewire-material::showcase.mail')->toHtml());
+    }
+}