Unify session action button styles: same padding, delete in red

- Add .session-copy CSS (was unstyled — browser default)
- .session-download/.session-copy: neutral gray, same padding 4px 12px
- .session-delete: red (--danger) instead of accent
- Bump 1.22.8, SW cache v39->v40

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simtrace-analyser-v39';
+const CACHE = 'simtrace-analyser-v40';
 
 const PRECACHE = [
     'index.html',