RTC: Fix stuck "Join" link in post list when lock expires

[shekharnwagh]

What?

Fixes two post list issues when RTC is enabled:

1. The "Join" action link gets stuck after the post lock expires
2. Checkboxes are hidden on locked post rows

Why?

PR #76322 conditionally rendered "Edit" or "Join" based on wp_check_post_lock() at page load. The heartbeat correctly clears the lock status text when the lock expires, but never updates the action link text back to "Edit".

Additionally, on WordPress < 7.0 (where the Gutenberg compat layer handles RTC), the CSS selectors that re-enable checkboxes on locked rows used .wp-locked, but core's list-tables.css uses tr.wp-locked which has higher specificity — so the override never took effect. This doesn't affect WordPress 7.0+ where the styles live directly in core's stylesheet with correct specificity.

How?

• Always render both "Edit" and "Join" as separate <span> elements in the row action link
• Toggle visibility via CSS using the .wp-locked class the heartbeat already manages
• Replace aria-label with screen-reader-text spans so the accessible name stays in sync with the visible text across lock state changes
• Fix CSS specificity for checkbox and inline edit overrides by adding the tr element selector to match core's tr.wp-locked

Same approach as the corresponding WordPress core fix: WordPress/wordpress-develop#11346

Testing Instructions

1. Enable RTC (Settings → Writing → Collaboration)
2. Log in as User A in one browser, open a post
3. Log in as User B in another browser, join the same post, then navigate to the Posts list
4. Verify the locked post shows "Join" link and "Currently being edited" text
5. Verify the checkbox is visible on the locked post row
6. Have User A leave the post
7. Wait for the lock to clear (~150s) on User B's post list
8. Verify "Currently being edited" text disappears and "Join" reverts to "Edit"

[github-actions]

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Co-authored-by: shekharnwagh <shekharnwagh@git.wordpress.org>
Co-authored-by: ellatrix <ellatrix@git.wordpress.org>
Co-authored-by: chriszarate <czarate@git.wordpress.org>

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[github-actions]

I just cherry-picked this PR to the wp/7.0 branch to get it included in the next release: 7ab515c