Playlist Track: Show upload spinner inline

[scruffian]

What?

Fixes the Playlist Track upload loading indicator so it appears inline with the uploaded track row instead of on the line above it.

Why?

The spinner was rendered as a sibling before the full-width track button, which caused it to sit on its own line during uploads.

How?

The spinner now renders inside the track button's flex row with scoped spacing, and the existing temporary blob upload unit test verifies the inline placement.

Testing Instructions

1. Open a post or page.
2. Insert a Playlist block.
3. Upload one or more audio files.
4. Confirm each loading indicator appears on the same line as its track.

Testing Instructions for Keyboard

1. Open a post or page.
2. Insert a Playlist block using the keyboard.
3. Use the block controls to upload an audio file.
4. Confirm the loading indicator appears inline with the uploading track and the track remains keyboard reachable.

Screenshots or screencast

N/A.

Use of AI Tools

Created with assistance from OpenAI Codex. The changes and tests were reviewed and run locally before opening this PR.

Checks run: npm run test:unit packages/block-library/src/playlist-track/test, npm run lint:js -- packages/block-library/src/playlist-track/edit.js packages/block-library/src/playlist-track/test/edit.js, npm run lint:css -- packages/block-library/src/playlist-track/style.scss, and git diff --check.

[github-actions]

Size Change: +243 B (0%)

Total Size: 7.71 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-library/index.min.js	357 kB	+6 B (0%)
build/styles/block-library/playlist-track/style-rtl.css	560 B	+32 B (+6.06%)	🔍
build/styles/block-library/playlist-track/style-rtl.min.css	529 B	+35 B (+7.09%)	🔍
build/styles/block-library/playlist-track/style.css	559 B	+30 B (+5.67%)	🔍
build/styles/block-library/playlist-track/style.min.css	528 B	+35 B (+7.1%)	🔍
build/styles/block-library/style-rtl.css	22.5 kB	+23 B (+0.1%)
build/styles/block-library/style-rtl.min.css	19 kB	+28 B (+0.15%)
build/styles/block-library/style.css	22.6 kB	+24 B (+0.11%)
build/styles/block-library/style.min.css	18.9 kB	+30 B (+0.16%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 0061ebd.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/31514184821
📝 Reported tests:

As a user I want to be able to add multiple close buttons so that users can close the overlay from different positions in /test/e2e/specs/site-editor/navigation-overlay-template-part.spec.js, passed after 1 failed attempt.

Error: expect(locator).toHaveCount(expected) failed

Locator:  getByRole('button', { name: 'Close' })
Expected: 2
Received: 1
Timeout:  5000ms

Call log:
  - Expect "toHaveCount" with timeout 5000ms
  - waiting for getByRole('button', { name: 'Close' })
    14 × locator resolved to 1 element
       - unexpected value "1"

    at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/site-editor/navigation-overlay-template-part.spec.js:198:33