Stop drawing each section's title twice on its page

Plan step 38 review: the frame's structural rule that kept a section's
own <h2> for screen readers (Tailwind's [&>section>h2]:sr-only before,
a showcase.css rule during the rewrite) was removed as dead weight
once every section had a real heading, so each page drew its title
twice, the <h1> and an identical headline under it. The <h2>s are
md-visually-hidden again, the frame's comment says why, and the
section page test asserts it. The Layout section's description said
"window size classes"; M3's term is breakpoints.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/views/showcase/sections/icons.blade.php

@@ -18,7 +18,7 @@
     }"
     x-intersect.once="load()"
 >
-    <h2 class="md-type-headline-md">Icons</h2>
+    <h2 class="md-visually-hidden">Icons</h2>
 
     <p class="md-type-body-md md-ink-variant">
         Every Material Symbol (Rounded, 400, grade 0) as <code>&lt;x-icon name="…" /&gt;</code>, outlined or <code>filled</code>,