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

tests/Browser/ContainmentTest.php

@@ -66,7 +66,7 @@ function overlayProbe()
 
 function containment()
 {
-    return visit('/material')->waitForEvent('networkidle')
+    return visit('/material/containment')->waitForEvent('networkidle')
         ->assertScript("document.readyState === 'complete' && typeof window.Alpine !== 'undefined' && typeof window.Livewire !== 'undefined'");
 }