Adds GetChangelog (all entries up to the running version, no seen-marker change)
and a Help menu 'What's new' item that reopens the changelog dialog on demand.
Embeds changelog.json (per-version EN + FR notes, curated from the release's
commits). GetWhatsNew compares the stored last-seen version with the running
build and returns the notes for every newer version, once, then advances the
marker. A dialog shows them in the UI language on the first launch after update.
Seeded with the 0.20.5 notes.
rouggy