feat(ENG-2810): prompt to migrate Markdown context tree to HTML

Add a dialog in the local web app that detects a legacy Markdown
context tree (via `migrate:run --dry-run`) and offers to convert
it to the 4.0 HTML format in place. On success it routes users to
the Changes tab to commit and push the new topics.

Dismissed projects still surface a "Migrate N topics to HTML"
button at the top of the Context tab, so users can return to the
dialog without losing the ability to act.

Dismissals persist in localStorage keyed by a djb2-hashed project
root, so raw paths don't leak into the key.