Make initial loading screen branding configurable via pxtarget.json instead of hardcoded msft class #11647

Describe the bug

In PR #9605 ("Branding: Add Microsoft logo to first loading screen"), the msft class was hardcoded directly into the template HTML in webapp/public/index.html:

<div id='loading' class="ui active dimmer">
    <div class="ui large main loader msft"></div>
</div>

Along with this change, theme/themes/pxt/globals/site.variables and theme/themes/pxt/elements/loader.overrides introduced:

@loaderMsftImageUrl: "../docs/static/orglogowide.png";
@loaderMsftImage: data-uri(@loaderMsftImageUrl);
.ui.loader.main.msft:after {
  background-image: @loaderMsftImage;
  background-size: 99%;
}

Why this is a problem

1. Trademark & Fork Compliance:
   Microsoft officially requires community-maintained forks and third-party targets to remove Microsoft logos and branding to prevent user confusion regarding affiliation.
2. Hardcoded Branding in Core:
   Because webapp/public/index.html is provided by pxt-core and hardcodes the class msft, every third-party MakeCode target displays the Microsoft logo on the initial loading screen by default.
3. Lack of Configuration:
   There is currently no target-level setting in pxtarget.json (such as appTheme.organizationWideLogo or appTheme.initialLoaderLogo) to configure the initial loading screen logo without relying on Microsoft-specific LESS variable names (@loaderMsftImageUrl).

Expected behavior

1. Third-party targets and community forks should be able to display their own organization logo on the initial loading screen (or use a neutral loader) without displaying Microsoft logos:
2. The class on the initial loader should be neutral (e.g., org-logo or initial-loader instead of msft).
   The initial loader image should be configurable via pxtarget.json (e.g., using appTheme.organizationWideLogo or a dedicated property), or via a neutral LESS variable (such as @loaderOrgImageUrl / @loaderInitialImageUrl).
3. If no organization logo is configured or if the target is not a first-party Microsoft target, the editor should not force Microsoft branding.