Chat panel (bottom-right) content stacks vertically / layout broken

[JohnMcLear]

The chat panel that pops up from the bottom-right of the pad lays out its contents vertically stacked rather than in the intended layout. Controls (header buttons, message list, input) are no longer positioned correctly against each other.

Reproduction

1. Open a pad.
2. Click the chat icon in the bottom-right.
3. Observe the panel: chat messages, header controls, and input are stacked in a way that doesn't match the intended layout.

Notes

• Appears to be pre-existing (not caused by the open fix(a11y): dialog semantics, focus management, icon labels, html lang #7584 a11y PR). Suspicion: one of the flex/grid properties on #chatbox or #chatbox-header was broken by a previous CSS refactor.
• Discovered while manual-testing fix(a11y): dialog semantics, focus management, icon labels, html lang #7584. Listed as a separate concern because the a11y PR didn't touch the chat panel layout.

Willing to share screenshots on request.

[JohnMcLear]

Attempting to shoe horn fix into #7584