Put the showcase in the app shell, with a page per section

The showcase is now an overview and one page per section behind a
grouped navigation rail, moved between with wire:navigate, instead of
one long page under a scrolling top bar. The switch exposed a WebKit
bug in the menu: inside a focusable region, WebKit moves focus out of
the closing popover before its toggle event, so Escape no longer
returned focus to the trigger. The menu now reads it on beforetoggle.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9NnLxnPp8vaaurb3Z1MFy

resources/views/showcase/section.blade.php

@@ -0,0 +1,29 @@
+{{-- One section of the showcase on a page of its own, with the way on to the next. The section's
+     own heading repeats the app bar's title, so only a screen reader hears it. --}}
+
+@extends('livewire-material::showcase.layout')
+
+@php
+    $keys = array_keys($sections);
+    $at = array_search($section, $keys, true);
+    $previous = $keys[$at - 1] ?? null;
+    $next = $keys[$at + 1] ?? null;
+@endphp
+
+@section('content')
+    <div class="mx-auto w-full max-w-6xl space-y-16 px-4 pt-2 pb-16 sm:px-6 [&>section>h2]:sr-only">
+        @include('livewire-material::showcase.sections.'.$section)
+
+        <nav aria-label="Sections" class="flex flex-wrap items-center justify-between gap-4 border-t border-divider pt-6">
+            @if ($previous)
+                <x-livewire-material::button variant="text" icon="arrow_back" :label="$sections[$previous]['title']" :link="route('livewire-material.section', $previous)" data-test="previous-section" />
+            @else
+                <x-livewire-material::button variant="text" icon="arrow_back" label="Overview" :link="route('livewire-material.showcase')" data-test="previous-section" />
+            @endif
+
+            @if ($next)
+                <x-livewire-material::button variant="tonal" icon-right="arrow_forward" :label="$sections[$next]['title']" :link="route('livewire-material.section', $next)" data-test="next-section" />
+            @endif
+        </nav>
+    </div>
+@endsection