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/text/message.blade.php

@@ -0,0 +1,32 @@
+{{-- The plain-text twin of html/message.blade.php: the same footer slot, no markup. --}}
+<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') }}. @lang('All rights reserved.')
+@endisset
+        </x-mail::footer>
+    </x-slot:footer>
+</x-mail::layout>