[SPARK-59085][CORE] Render the web UI's inline (kill), (hold), and (resume) links as buttons

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR renders the web UI's inline (kill), (hold), and (resume) text links as buttons, using the btn btn-sm btn-outline-* style already used elsewhere in the UI.

BEFORE (Apache Spark 5.0.0-SNAPSHOT)

AFTER (This PR)

• Master UI application and driver tables: these controls are already wrapped in POST forms, so their anchors become <button type="submit">.
• Stage and job tables, and the jobs page summary: these must stay GET links (SPARK-6846: the YARN AM proxy does not forward POST), so they remain anchors and gain role="button".
• kill uses btn-outline-danger and hold/resume use btn-outline-secondary, separating the destructive control from the reversible one.
• Labels drop the parentheses that marked them as links: (kill) -> Kill, (hold) -> Hold, (resume) -> Resume.
• Because a submit button posts its own form, webui.js no longer has to submit the form on the anchor's behalf. The two otherwise identical confirmation handlers collapse into one that works for both links and buttons (22 lines -> 8).
• kill-link and confirm-link remain as JS and test hooks but no longer carry colors; the Bootstrap button variant owns those now.
• docs/web-ui.md and docs/spark-standalone.md are updated to match the new labels.

Why are the changes needed?

These controls perform actions -- killing an application, stage, or job, and holding or resuming an application -- but render as small parenthesized secondary-colored text, which reads as body copy rather than as something clickable, and gives a hit target only as wide as the word.

This also aligns them with the button style the UI has been converging on. SPARK-59066 and SPARK-59067 recently moved the History Server's Download control and the SQL execution page's Download control to btn btn-sm btn-outline-secondary; this PR applies the same style to the remaining inline controls.

Converting the Master UI's POST controls to real submit buttons also makes them reachable by keyboard, which <a href="#"> was not, and removes the JS workaround that submitted the form for them.

Does this PR introduce any user-facing change?

Yes, this is a visual change to the web UI. The controls listed above now render as buttons and their labels lose the surrounding parentheses.

One behavior change is worth calling out: the Master UI's kill, hold, and resume controls are now native submit buttons, so if webui.js fails to load, clicking one posts the form without showing the confirmation prompt. Previously the <a href="#"> did nothing at all without JS. This matches the driver UI's kill and hold controls, which are GET links and have always navigated without a prompt when JS is unavailable.

How was this patch tested?

• Existing coverage is unaffected: UISeleniumSuite locates these controls by the kill-link and confirm-link class names, which are preserved, and MasterWebUISuite asserts on the rendered app/kill/, app/hold/, and app/resume/ form actions, which are unchanged.
• core/compile, core/scalastyle, and dev/lint-js pass.
• Verified manually

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

Generated-by: Claude Opus 5

[dongjoon-hyun]

Could you review this PR too, @peter-toth ?

[dongjoon-hyun]

Thank you always, @peter-toth !

[dongjoon-hyun]

Merge Summary:

• merged into master 598ca22
• merged into branch-4.x d350939

Posted by merge_spark_pr.py