feat: themed scrollbars and chiseled thumbnail chips

- Replace browser scrollbars with slim 6px overlay pills that tint to
  primary on hover (transparent track, Firefox + WebKit).
- Rework the thumbnail badge family: drop the mismatched white ring
  halo for a 1px dark frame + inset top highlight, and swap rounded-
  full pills for rounded-sm chips so the ornaments match the pixel-art
  aesthetic used elsewhere in the app.
- Soften the ActiveHeapCard desert backdrop so the fanned thumbnails
  read on top.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/heaps/ActiveHeapCard.tsx

@@ -88,6 +88,7 @@ export function ActiveHeapCard() {
           backgroundPosition: 'center bottom',
           backgroundRepeat: 'no-repeat',
           imageRendering: 'pixelated',
+          opacity: 0.7,
         }}
       >
         {visible.length === 0 ? (