Add Custom Pages open action

[paullizer]

Summary

• Add an Open action to the Admin Settings Custom Pages table for enabled static and Python-backed pages.
• Route the action through encoded /custom/<slug> URLs so existing Custom Pages feature gating, enabled state, access-level authorization, role checks, and .html alias compatibility remain server-owned.
• Disable unavailable Open actions with explanatory tooltip copy when Custom Pages is not runtime-enabled or a page is disabled.
• Update Custom Pages docs, functional wiring coverage, UI contract coverage, and bump the app version to 0.250.106.

Validation

• node --check application\single_app\static\js\admin\admin_custom_pages.js
• python functional_tests\test_custom_pages_wiring.py
• python -m pytest ui_tests\test_admin_custom_pages_open_action.py -q
• git -c core.whitespace=blank-at-eol,blank-at-eof,space-before-tab,cr-at-eol diff --check

Fixes #951