[codex] Improve Remote UI agent messaging and iPad header

[firewalker06]

Summary

• simplify the Remote UI agent workflow and update the related Remote UI docs/status notes
• render assistant messages and run summaries as scoped markdown while preserving chat layout constraints
• add iPad standalone header spacing, iOS safe-area handling, and a stable subtitle icon slot so refresh state does not shift header text

Root Cause

The iPad obstruction was not the normal status-bar safe area. It was the floating standalone/window control drawn over the left edge of the app, which is not represented by safe-area-inset-top. The header now detects iPad-like standalone mode and reserves left-side space for that control. The subtitle shift came from only inserting the refresh icon while polling; the subtitle now keeps a persistent icon slot.

Validation

• bundle exec ruby test/remote_server_test.rb
• bin/test
• git diff --check
• local Chrome/CDP simulation of iPad standalone mode confirmed ipad-standalone applied, --ipad-header-control-space: 112px, header padding 124px, and logo left edge at x=133 on a 773px-wide viewport