feat(rail): voicings rail polish — 4 guitar shapes, 2x2 mini piano, no play buttons, dark scrollbars (task L-70)

Dashboard voicings rail: guitar galleries capped to 4 shapes
(recommended-first, then open, then movable by base fret; prop-gated so
the Knowledge Center and chord modal keep all shapes); piano voicings
render as a 2x2 grid of size="mini" (scale 0.60) keyboards under the
rail's dense path only. Play buttons removed globally from
VoicingBrowser per the user ("leave them off, better not") — the
Knowledge Center + modal render byte-identical minus the buttons.
A scoped .dark-scroll utility (thin, #2a2a2a on transparent) dresses
the dashboard scrollers. Critic PASS (combined gate: byte-identity
proven, zero play buttons everywhere, contract intact).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/index.css

@@ -6,3 +6,19 @@ body {
   color: #f5f5f5;
   font-family: system-ui, -apple-system, sans-serif;
 }
+
+/* Thin, dark, overlay-feel scrollbars for the jam dashboard's scrollers
+   (dashboard-polish.md §4). Scoped (a class), NOT global — the KnowledgeDock /
+   Debug / Tuner / Settings keep the OS default. Raw hex is unavoidable here:
+   ::-webkit-scrollbar pseudo-elements are not reachable by Tailwind utilities;
+   the values mirror the design tokens (border #2a2a2a, panel #1a1a1a). */
+.dark-scroll {
+  scrollbar-width: thin;                    /* Firefox */
+  scrollbar-color: #2a2a2a transparent;     /* thumb = border token · track transparent */
+}
+.dark-scroll::-webkit-scrollbar { width: 8px; height: 8px; }        /* WebKit/Blink */
+.dark-scroll::-webkit-scrollbar-track { background: transparent; }
+.dark-scroll::-webkit-scrollbar-thumb {
+  background: #2a2a2a; border-radius: 4px;  /* border token */
+}
+.dark-scroll::-webkit-scrollbar-thumb:hover { background: #3a3a3a; }