fix(uploads): refuse an unsupported video before uploading it
Conflict resolved against #621 in src/web/public/image-input.js (the
_promptAttachKind doc comment only): #621's comment is kept with #622's
clause that a video MIME counts only for the containers the server
admits; #622's video MIME check and #621's document branch both stay.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>