Issue #3526909: Remove reference to non-existent progress.gif in Claro and default_admin

Removes the url(../../../../misc/progress.gif) from .ui-progressbar .ui-progressbar-value. The file does not exist in core, so the reference 404s. Nothing else in the rule depends on it, and the rendered result is unchanged.

Also fixes the same line in default_admin, which appears to have inherited Claro's jQuery UI styles.

Source .pcss.css edited and yarn build:css run to regenerate the compiled CSS.

I used Claude to help set up my local environment and review the diff. The change is a one-line removal I verified myself.

Closes #3526909