Fix Control Center left nav endpoint check

[Bionic711]

Summary

• Fix the Control Center full-sidebar endpoint check so the page-local left nav renders on /admin/control-center.
• Preserve regular Admin access when ControlCenterAdmin enforcement is disabled.
• Add functional regression coverage, fix documentation, release notes, and bump the app version to 0.250.052.

Validation

• git diff --check
• c:/Repos/simplechatmsft/.venv/Scripts/python.exe -m py_compile application/single_app/config.py functional_tests/test_control_center_left_nav_endpoint.py
• c:/Repos/simplechatmsft/.venv/Scripts/python.exe -m compileall -q application/single_app
• c:/Repos/simplechatmsft/.venv/Scripts/python.exe functional_tests/test_control_center_left_nav_endpoint.py
• c:/Repos/simplechatmsft/.venv/Scripts/python.exe scripts/check_xss_sinks.py --full-file application/single_app/templates/_sidebar_nav.html

Notes

• Playwright UI tests were not run because this PR-prep context did not have a confirmed running local app plus authenticated admin storage state.
• VS Code diagnostics still show pre-existing inline-style warnings in _sidebar_nav.html; this PR does not introduce new inline styles or frontend script sinks.

Fixes #1009