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

tests/Feature/ShowcaseTest.php

@@ -51,3 +51,20 @@ it('lists the symbol names for the icon search', function () {
         ->assertJsonFragment(['calendar_month'])
         ->assertJsonCount(count(SvgFile::symbolNames()));
 });
+
+it('previews an error page through the exception handler', function () {
+    $this->withoutVite()
+        ->get('/material/errors/404')
+        ->assertNotFound()
+        ->assertSee('Page not found');
+
+    $this->get('/material/errors/418')->assertNotFound();
+});
+
+it('previews a sample mail in the package theme', function () {
+    $this->get('/material/mail')
+        ->assertOk()
+        ->assertSee('class="button button-primary"', false)
+        ->assertSee('border-radius: 9999px', false)
+        ->assertSee('Unsubscribe');
+});