Revert "MUL-6385: feat(chat): add editable live HTML preview (#7188)" (#7584)

This reverts commit 0c9f50d1f47a316e75d2accb1255ebeed2a0f818.

The feature was merged after a code-quality review, but we did not settle the
product question first: what problem the editable live preview actually solves.
Reverting to keep main aligned with a decision we can state, rather than
carrying a surface we are not sure we want to maintain.

Pure revert — restores the touched files to their exact pre-merge state, with
no dangling references left behind.

Co-authored-by: J <bohan@devv.ai>
Co-authored-by: multica-agent <github@multica.ai>