feat: add a Who's Online widget to the Network Admin dashboard

[josephfusco]

Part of the network presence stack, merged bottom-up:

1. feat: add a network-wide presence summary table #332 summary table
2. feat: push each site's online set into the network summary #333 write path
3. feat: read the network summary as a capped snapshot #334 read path
4. feat: add an Online column to the Network Sites list #335 Sites list column
5. feat: add an Online view and column to the Network Users list #336 Users list
6. feat: add a Who's Online widget to the Network Admin dashboard #337 network dashboard widget (this one)

---

Last surface, and the only one that ticks on a heartbeat rather than rendering once.

Part of #298
Closes #325

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5, Claude Opus 5
Used for: Assisting with design, implementation, and testing

[github-actions]

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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 a05a58c978d92172c69b5245efa1e84845cd77ea. Auto-updates when you push.}

[josephfusco]

Review findings from #313 that land here are #325, closed by this one, and #331, which is not.

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.76%. Comparing base (8cee7a2) to head (a05a58c).

Additional details and impacted files

@@                       Coverage Diff                        @@
##             feature/network-users-list     #337      +/-   ##
================================================================
+ Coverage                         96.73%   96.76%   +0.03%     
- Complexity                          188      208      +20     
================================================================
  Files                                17       18       +1     
  Lines                              2418     2504      +86     
================================================================
+ Hits                               2339     2423      +84     
- Misses                               79       81       +2     

Flag	Coverage Δ
multisite	96.76% <97.67%> (+0.03%)	⬆️
phpunit	79.55% <0.00%> (-2.83%)	⬇️

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