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/500.blade.php

@@ -0,0 +1,9 @@
+{{-- Server error. Nothing from the exception is shown: its message is for the log, not the visitor. --}}
+
+@extends('errors::minimal')
+
+@section('title', __('Server Error'))
+@section('code', '500')
+@section('shape', 'soft-burst')
+@section('headline', __('Something went wrong'))
+@section('message', __('An error on our side stopped this page from loading. Please try again in a moment.'))