Theme Directory: Indicate upload progress on the theme upload form

[obenland]

Fixes #7852

As noted in #438, the Theme Directory upload page has no visible "upload in progress" state on its submit button in the new (2024) theme — the old theme relied on button[disabled] styles that no longer exist.

This change makes the upload state explicit, following the pattern already used by the Plugin Directory upload form (data-uploading-label):

• When the form submission proceeds, the submit button is disabled and its label changes from Upload to Uploading….
• A #upload_button:disabled style greys the button out (opacity: 0.6, cursor: not-allowed), matching the disabled-button treatment in wporg-plugins-2024. Since neither wporg-parent-2021 nor wporg-mu-plugins ship :disabled button styles, the style is attached to an inline-only wporg-themes-upload handle via wp_add_inline_style() so it works in both the old and new theme; the submit handler is attached the same way via wp_add_inline_script() (footer, after the form markup).
• The previous inline onsubmit handler disabled the button unconditionally — including when the 2FA revalidation script cancels the submission to show its modal, which left the button stuck disabled if the modal was dismissed. The new handler defers until after all submit handlers have run and checks event.defaultPrevented, so the button state only changes when the upload is actually underway. Once revalidation completes and the form is re-submitted via requestSubmit(), the state is applied as usual.

The button markup keeps the id="upload_button" class="button" prefix so the theme-upload-form block in wporg-themes-2024 continues to rewrite it into a wp-block-button__link; the new data-uploading-label attribute is carried through that rewrite (verified against the block's preg_replace).

Verified in the local theme-directory wp-env environment (Chromium): the plain submit path flips the button to disabled + "Uploading…", and — with the production wporg-two-factor revalidation script injected (the local env stubs it out) — the button stays "Upload" while the revalidation modal is open, remains usable if the modal is dismissed, and flips to "Uploading…" once revalidation completes and the form re-submits.

🤖 Generated with Claude Code

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