Merge branch 'main' into task/146

main's 830ea832 dropped fork-and-send, taking the `onForkSend` prop and the
composer's send split button with it — the same declarations this branch edits
to widen `onSteer` with the steered draft's blocks. Three files conflicted:
message-input.tsx, chat-input.tsx and conversation-shell.tsx, all on that
adjacent prop block. Each resolves the same way: keep main's removal of
fork-and-send, keep the widened `onSteer(text, blocks?)`.

Nothing else needed hand-resolving; the steering chain, the i18n removals and
the backend guard all merged clean, and no fork-send reference survives.