Improve scanning UX with skeleton loaders

[ieliofficial]

Currently during the 1.2s "scanning" delay, the UI shows "Scanning…" text. More polished apps show skeleton loaders to indicate progress.

Requirements:

1. Add skeleton loader components that appear during scanning state
2. Skeletons should mimic the final layout (problem category boxes, etc.)
3. Keep existing accessibility - skeletons should be hidden from screen readers
4. Ensure the UI is still usable during scan