Right panel: a floating panel can cover the whole chat with focus left in the hidden composer; no resize while floating #718

After #691, below the docking cutoff the right panel floats over the chat. At small widths (e.g. 720pt with the sidebar open) the overlay is as wide as the chat column, so nothing of the chat is visible while keyboard focus stays in the hidden composer and typing goes there unseen; there is no Escape or focus handling for the panel, and the floating panel has no resize handle. Separately, with the panel closed, the chat header's toggles are still clipped at the window edge in very narrow windows (left out of #644 by scope). Expected: focus moves into the panel when it floats and returns on close; Escape closes it; a drag edge while floating; the header compacts further or scrolls when the chat column is narrower than its compact header. Owner: crates/ui/src/shell.rs (floating branch, focus), crates/ui/src/chat/mod.rs (header).