The chat run settings panel was fixed at 17rem. It now uses the same drag
handle as the sidebar, on its left edge, between 248px and 560px and capped
at 40% of the window. The width persists and syncs across tabs.
Reuses PanelResizeHandle and createPanelWidthStore, so behaviour matches the
sidebar exactly. The panel width key joins the preference reset list.
The system prompt overflow check now runs off a ResizeObserver attached
through a callback ref. A drag changes the width through a custom property
without re-rendering, and the collapsible section unmounts the textarea, so
a stored observer would miss both.
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