ChatPanel._sync called scroll_end on every WS event (every stream_delta),
yanking the view back to the bottom on each token — impossible to scroll up
and read earlier messages during a response. Stick-to-bottom: capture
is_vertical_scroll_end before any DOM change and only scroll_end when the
user was already at the bottom; auto-follow resumes when they scroll back.

Add app-level keyboard bindings so the chat scrolls without leaving the
input box: Ctrl+Shift+Up/Down (one line), Ctrl+End (jump to bottom). These
keys are not claimed by TextArea (it uses shift+up/down for select and
ctrl+e/end for line end), so they bubble from the input to the app.