[LIVY-1088] Fix Web UI progress bar and select styling blocked by default CSP

[idzikovsky]

What changes were proposed in this pull request?

The default livy.server.http.header.Content-Security-Policy does not allow
inline style attributes or data: images (img-src 'self'), which breaks two
parts of the Web UI:

• Statement progress bars: progressBar() set the width through an inline
  style attribute, which the browser ignores. The width is now stored in a
  data-width attribute and applied through CSSOM (element.style.width) by
  the new applyProgressBarWidths() helper, called from loadStatementsTable().
• Bootstrap select.custom-select elements: their arrow is a data: SVG
  background image, which is blocked. They now fall back to the browser's
  native <select> appearance.

The default CSP is unchanged.

Screenshot of the Livy UI before the fix:

And after the fix:

How was this patch tested?

Manually checked the session page and sessions list in a browser with the
default CSP enabled.

Was this patch authored or co-authored using generative AI tooling?

Yes.

Generated-by: GitHub Copilot CLI (claude-opus-5.5)

[idzikovsky]

This fix will be merged in the scope of #563