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/mail/html/message.blade.php

@@ -0,0 +1,35 @@
+{{-- Every Markdown mail passes through here. The framework's message, with the footer open to
+     the mail: a `<x-slot:footer>` inside `<x-mail::message>` replaces the default line — for the
+     sender's address, a privacy link, or an unsubscribe link on mail that is not transactional.
+     Keep the Markdown below unindented: four spaces make a code block. --}}
+<x-mail::layout>
+{{-- Header --}}
+<x-slot:header>
+<x-mail::header :url="config('app.url')">
+{{ config('app.name') }}
+</x-mail::header>
+</x-slot:header>
+
+{{-- Body --}}
+{!! $slot !!}
+
+{{-- Subcopy --}}
+@isset($subcopy)
+<x-slot:subcopy>
+<x-mail::subcopy>
+{!! $subcopy !!}
+</x-mail::subcopy>
+</x-slot:subcopy>
+@endisset
+
+{{-- Footer --}}
+<x-slot:footer>
+<x-mail::footer>
+@isset($footer)
+{!! $footer !!}
+@else
+© {{ date('Y') }} {{ config('app.name') }}. {{ __('All rights reserved.') }}
+@endisset
+</x-mail::footer>
+</x-slot:footer>
+</x-mail::layout>