1.0.16

Add a homepage/navbar download widget and desktop "what's new" changelog

New data/changelog.js is the single source of truth for desktop release
notes (also drives desktopApp.js's version, replacing a version constant
that had drifted 6 releases behind).

- Homepage hero and navbar: a compact "Download for Windows" button plus
  a version picker (DownloadWidget), hidden inside the desktop app itself.
- /download page: same version picker, plus a "Recent updates" section
  listing the last 3 releases' notes.
- Desktop app: the "Update available" popup now lists what's in that
  update; a new "What's new in vX.Y.Z" popup appears once, right after a
  restart lands on a new version (skipped on a genuine first install).

Verified all of the above with a temporary window.fq stub in the browser
(update popup, restart "what's new" popup, and that the download UI
hides correctly when isDesktop() is true), then reverted the stub.