Rewrite the showcase's Sliders section without Tailwind

Plan step 38: single-column groups move onto <x-stack>, per-breakpoint
ones onto <x-grid>, and the "Vertical" row's shared height (M3's
second orientation needs one on the wrapper, which the label and hint
share) moves to a showcase-slider-vertical class — <x-slider> forwards
only class, style and wire:key to that wrapper, so a data attribute
would have been dropped silently.

Every #sliders id and data-bound span the Chromium SliderTest reads is
unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/css/showcase.css

@@ -456,3 +456,9 @@
 .showcase-w-narrow {
     max-width: 320px;
 }
+
+/* <x-slider> forwards only class, style and wire:key to its wrapper, which the label and hint
+   share with the track — so the vertical demo's shared height is a class too. */
+.showcase-slider-vertical {
+    height: 256px;
+}