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/showcase/mail.blade.php

@@ -0,0 +1,40 @@
+{{-- The showcase's sample Markdown mail: every part the theme styles — heading, prose, bold,
+     a list, the button, a panel, a table, subcopy and the footer slot. Markdown, so it stays
+     unindented. --}}
+<x-mail::message>
+# Your export is ready
+
+Hello Anna, the report you asked for on **Monday** has finished. It holds three files and stays available for seven days.
+
+<x-mail::button :url="url('/')">
+Download the export
+</x-mail::button>
+
+<x-mail::panel>
+Files are deleted automatically once they expire. Download them before **20 September** to keep a copy.
+</x-mail::panel>
+
+<x-mail::table>
+| File | Rows | Size |
+|:-----|-----:|-----:|
+| customers.csv | 1,204 | 184 KB |
+| orders.csv | 8,930 | 1.2 MB |
+| summary.pdf | — | 96 KB |
+</x-mail::table>
+
+What happens next:
+
+1. Open the export from the button above.
+2. Check the summary before you share it.
+
+Thanks,<br>
+{{ config('app.name') }}
+
+<x-slot:subcopy>
+If the button does not work, paste this address into your browser: [{{ url('/') }}]({{ url('/') }})
+</x-slot:subcopy>
+
+<x-slot:footer>
+© {{ date('Y') }} {{ config('app.name') }} · [Privacy]({{ url('/') }}) · [Unsubscribe]({{ url('/') }})
+</x-slot:footer>
+</x-mail::message>