Update the site for MPCkit 0.5.3

## What changed

- `public/index.html`:
  - the badge says Version 0.5.3;
  - the Download note says "The 0.5 releases are betas for testing", as the app's README does, so it stays right for later 0.5.x releases;
  - the image descriptions name pads "A3" and "A1", and the image sizes match the new screenshots.
- `public/images/pads.png`, `sequences.png`: new screenshots of 0.5.3 from the Windows build, with the window title "MPCkit" and the new "A1" pad labels. The local path in the status bar ("Loaded C:\Users\…") was painted out.

## Notes

- Source links and downloads are unchanged; they move to GitHub in a later PR.

## Testing

- Served `public/` locally and checked desktop and mobile screenshots.