web: diff-line review threads — comment and reply #44

mr diff-comment (new threads and --reply) exists over SSH and the JSON API, and the iOS app now supports replying on threads — but the web renders threads read-only-ish: the Parity matrix rows "comment on a diff line: web no". The web is now the only surface that cannot participate in inline review.

For CLI <=> web <=> iOS parity at v1.0.0, the web diff view needs comment-on-line and reply forms dispatching mr diff-comment, plus resolve/unresolve controls if any are missing.

Ref #11