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/index.html

@@ -230,8 +230,23 @@
         <p id="empty-state" class="empty">No events logged for this day.</p>
       </section>
     </main>
+
+    <footer class="app-footer">
+      <button type="button" id="changelog-btn" class="linklike">Changelog</button>
+    </footer>
     </div><!-- /#app -->
 
+    <dialog id="changelog-dialog">
+      <form method="dialog" id="changelog-form">
+        <h3>Changelog</h3>
+        <ul id="changelog-list" class="changelog-list"></ul>
+        <p id="changelog-empty" class="empty" hidden>No changelog available.</p>
+        <menu>
+          <button value="close">Close</button>
+        </menu>
+      </form>
+    </dialog>
+
     <dialog id="settings-dialog">
       <form method="dialog" id="settings-form">
         <h3>Puppy settings</h3>