Scrollbars: one 12px style for every pane

- the APDU list's 12px rule is folded into the global scrollbar style, so
  the side panel, APDU detail pane, dialogs (file-diff body), session
  lists/grid and the hex views' horizontal bars all use the same 12px bar
  with a 6px rounded thumb — no per-element overrides remain to drift,
  and the horizontal bars get an explicit height instead of the browser
  default
- Node test pins the global rule (12px width/height, 6px thumb radius) and
  the absence of per-element overrides; 527 Python + 55 Node tests
- SW cache simtrace-analyser-v75 (cosmetic, no version bump)

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simtrace-analyser-v74';
+const CACHE = 'simtrace-analyser-v75';
 
 const PRECACHE = [
     'index.html',