fix: preserve focus across heartbeat re-renders in dashboard widgets

[i-am-chitti]

Description

• Capture focus before container.html() swaps in Active Posts and Who's Online, restore it after - falling back to the container if the focused item no longer exists.
• Add data-post-id to Active Posts rows (Who's Online already had data-user-id); both containers get tabindex="-1" as the fallback focus target.
• Add e2e coverage for focus preservation across a heartbeat re-render in both widgets.

Fixes #265.

Test plan

• npx playwright test --config tests/e2e/playwright.config.js tests/e2e/presence-widgets.test.js

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Investigating the focus-loss cause and writing the e2e tests

[github-actions]

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Props iamchitti, joefusco.

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[github-actions]

▶ Preview in WordPress Playground

Boots a fresh WordPress with this PR's presence-api build, seeds 5 demo users, and drops you on the dashboard.

_{Stress-test variant: 40 demo users · Built from 8fdedf8787aa11f0ed7a06282c9c75337523ca81. Auto-updates when you push.}

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.87%. Comparing base (e438faa) to head (8fdedf8).

❌ Your patch check has failed because the patch coverage (50.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #271   +/-   ##
=========================================
  Coverage     75.87%   75.87%           
  Complexity      176      176           
=========================================
  Files            14       14           
  Lines          1832     1832           
=========================================
  Hits           1390     1390           
  Misses          442      442           

Flag	Coverage Δ
multisite	75.87% <50.00%> (ø)
phpunit	74.56% <50.00%> (ø)

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[josephfusco]

Nice work!