Add a Changelog button at the bottom of the page

Opens a dialog listing the loaded build's full changelog, grouped by
date. Fetched cache-first through the service worker, so the list always
matches the running version; the update banner still covers what a
waiting build adds.

src/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-07-12", "text": "Browse the full changelog from the bottom of the page" },
   { "date": "2026-07-12", "text": "Finer-grained y-axis on the daily counts chart" },
   { "date": "2026-07-12", "text": "The update banner now lists what changed in the new version" },
   { "date": "2026-07-12", "text": "Fixed adding an exercise on iPhone: stale app updates, and the keyboard's Go key discarding the input" },