When the composer contentEditable is focused, PageUp/PageDown propagate
to the chat-thread scroll container and cause layout breakage: large
blank area at the bottom, content shifted left, sidebar pushed
off-screen.
Intercept both keys in handleEditorKeyDown and preventDefault() so the
browser's native scroll behaviour stays inside the single-line
composer where these keys have no text-editing purpose.
Fixes#49978
f7bc26e40e