Side panel: scrollbar matches the APDU list's width

- the side panel had no scrollbar rule of its own, so it fell back to the
  global 6px default while the APDU list uses 12px; the two now share one
  rule (width 12px, thumb radius 6px = half the bar), so they cannot drift
  apart again — the other panes keep the thin global bar
- Node test pins the shared rule (and the 6px global default)
- SW cache simtrace-analyser-v74 (cosmetic, no version bump)

frontend/tests/style.test.js

@@ -0,0 +1,22 @@
+// Scrollbar styling: the APDU list and the side panel share one rule, so
+// their widths cannot drift apart (the side panel used to fall back to the
+// global 6px bar while the list uses 12px).
+
+const { test } = require('node:test');
+const assert = require('node:assert');
+const { loadSrc } = require('./helpers.js');
+
+const src = loadSrc();
+
+test('the APDU list and the side panel share the 12px scrollbar rule', () => {
+	const rule = /\.apdu-list::-webkit-scrollbar,\s*\n\.side-panel::-webkit-scrollbar \{ width: 12px; \}/;
+	assert.match(src, rule);
+	// the thumb radius pairs with the width (6px = half of 12px)
+	assert.match(
+		src,
+		/\.apdu-list::-webkit-scrollbar-thumb,\s*\n\.side-panel::-webkit-scrollbar-thumb \{ border-radius: 6px; \}/);
+});
+
+test('the global scrollbar default stays thin for the other panes', () => {
+	assert.match(src, /::-webkit-scrollbar \{ width: 6px; \}/);
+});