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

routes/showcase.php

@@ -1,6 +1,7 @@
 <?php
 
 use Illuminate\Support\Facades\Route;
+use NoNameWeb\LivewireMaterial\Http\Controllers\ShowcasePageController;
 use NoNameWeb\LivewireMaterial\Http\Controllers\ShowcaseSymbolController;
 
 Route::view('/', 'livewire-material::showcase.index')->name('showcase');
@@ -11,3 +12,8 @@ Route::get('symbols/{style}/{name}.svg', [ShowcaseSymbolController::class, 'show
 Route::view('shell/{page?}', 'livewire-material::showcase.shell')
     ->whereIn('page', ['inbox', 'starred', 'sent', 'drafts', 'travel', 'receipts'])
     ->name('shell');
+
+Route::get('errors/{code}', [ShowcasePageController::class, 'error'])
+    ->whereIn('code', ['401', '402', '403', '404', '419', '429', '500', '503'])
+    ->name('error');
+Route::get('mail', [ShowcasePageController::class, 'mail'])->name('mail');