refactor: replace hardcoded admin/online room string with helper

[i-am-chitti]

Description:

Adds wp_presence_admin_room(), mirroring wp_presence_post_room(), and uses it in place of the 'admin/online' literal across heartbeat.php, user-list.php, admin-bar.php, lifecycle.php, and the Who's Online widget (which also drops its own ROOM constant, since a class constant can't hold a function call's return value).

Related:

• admin/online room name is hardcoded in eight places #154

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Implementation across all six files; reviewed and verified by me.

[github-actions]

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Props iamchitti.

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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 b1c4927cde56b94f36229b2d541f153cc5febb92. Auto-updates when you push.}

[codecov]

Codecov Report

❌ Patch coverage is 30.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.11%. Comparing base (3406134) to head (b1c4927).
⚠️ Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
includes/heartbeat.php	33.33%	2 Missing ⚠️
includes/user-list.php	0.00%	2 Missing ⚠️
includes/admin-bar.php	0.00%	1 Missing ⚠️
includes/functions.php	0.00%	1 Missing ⚠️
...s/widgets/class-wp-presence-widget-whos-online.php	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #176      +/-   ##
============================================
- Coverage     29.13%   29.11%   -0.02%     
  Complexity      164      164              
============================================
  Files            15       15              
  Lines          1840     1841       +1     
============================================
  Hits            536      536              
- Misses         1304     1305       +1     

Flag	Coverage Δ
phpunit	29.11% <30.00%> (-0.02%)	⬇️

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