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/style.css

@@ -695,6 +695,40 @@ input.switch:checked::after { transform: translateX(18px); }
   opacity: 0.92;
 }
 .update-changelog[hidden] { display: none; }
+
+.app-footer {
+  text-align: center;
+  padding: 4px 0 24px;
+}
+.app-footer .linklike { color: var(--muted); font-size: 0.8rem; }
+
+.changelog-list {
+  list-style: none;
+  margin: 0 0 12px;
+  padding: 0;
+  max-height: 60vh;
+  overflow-y: auto;
+}
+.changelog-date {
+  color: var(--muted);
+  font-size: 0.75rem;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  margin: 10px 0 4px;
+}
+.changelog-date:first-child { margin-top: 0; }
+.changelog-entry {
+  font-size: 0.9rem;
+  line-height: 1.45;
+  padding: 3px 0 3px 14px;
+  position: relative;
+}
+.changelog-entry::before {
+  content: "•";
+  position: absolute;
+  left: 2px;
+  color: var(--accent);
+}
 .update-banner-msg { font-size: 0.9rem; font-weight: 600; }
 .update-banner-actions { display: flex; gap: 8px; }
 .update-banner-btn {