feat: refresh the Online users list on each heartbeat

[josephfusco]

Fixes #562

Rows come from core's list table with the page's query swapped into the heartbeat request, which is the part most likely to be wrong; the network view is only unit tested, and "N items" and the page links don't refresh.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Assisting with the heartbeat handler, client swap and tests.

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Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.28%. Comparing base (4218200) to head (2e693b1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #604   +/-   ##
=========================================
  Coverage     95.28%   95.28%           
  Complexity      191      191           
=========================================
  Files            24       24           
  Lines          3054     3077   +23     
=========================================
+ Hits           2910     2932   +22     
- Misses          144      145    +1     

Flag	Coverage Δ
multisite	95.28% <95.65%> (+<0.01%)	⬆️
phpunit	70.03% <91.30%> (+0.16%)	⬆️

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