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

resources/views/error-pages/errors/503.blade.php

@@ -0,0 +1,22 @@
+{{-- Service unavailable, usually maintenance mode. A message the app passed (`abort(503, '…')`) is
+     the sentence; maintenance mode's own "Service Unavailable" is not a message, so it is not.
+
+     `php artisan down --render="errors::503"` renders this once, in the console, with no
+     `$exception` and no request — so the one action reloads whatever address the visitor is
+     on, in the browser, rather than a URL decided here. --}}
+
+@extends('errors::minimal')
+
+@php
+    $reason = isset($exception) ? $exception->getMessage() : '';
+@endphp
+
+@section('title', __('Service Unavailable'))
+@section('code', '503')
+@section('shape', 'puffy')
+@section('headline', __('We’ll be right back'))
+@section('message', $reason !== '' && $reason !== 'Service Unavailable' ? __($reason) : __('We’re making some improvements. Please check back soon.'))
+
+@section('actions')
+    <x-button :label="__('Try again')" variant="filled" size="md" onclick="location.reload()" />
+@endsection