Disable chat checkbox doesn't visually disable dependent 'Chat always on screen' control

[JohnMcLear]

In the pad Settings dialog, checking Disable chat disables the underlying behaviour but does not visually disable the dependent Chat always on screen control. The checkbox still looks enabled / clickable to the user, which is misleading UX.

Reproduction

1. Open a pad.
2. Open Settings (toolbar cog icon).
3. Under "My View": tick the Disable chat checkbox.
4. Note that Chat always on screen is functionally disabled, but visually renders as if enabled (no greyed-out label, checkbox still looks interactive).

Expected

When Disable chat is checked, the dependent "Chat always on screen" row should be visually disabled: greyed label, disabled attribute on the checkbox, etc.

Notes

• Pre-existing (not related to the open fix(a11y): dialog semantics, focus management, icon labels, html lang #7584 a11y PR). The sibling-disable logic never existed; disabling is implicit in the JS but not reflected in the DOM/CSS.
• Small fix: toggle disabled attribute on #options-chatandusers in the #options-disablechat change handler, or wrap both in a fieldset that toggles disabled collectively.

Discovered while manual-testing #7584.