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/app.js

@@ -1916,6 +1916,51 @@
     settingsDialog.close();
   });
 
+  // ---------- changelog dialog ----------
+  // Shows the *loaded* build's full changelog: the plain URL is served
+  // cache-first by the controlling service worker, so the list always matches
+  // the version actually running (the update banner handles what's newer).
+  const changelogDialog = document.getElementById("changelog-dialog");
+  const changelogList   = document.getElementById("changelog-list");
+  const changelogEmpty  = document.getElementById("changelog-empty");
+
+  function formatChangelogDate(iso) {
+    const [y, m, d] = (iso || "").split("-").map(Number);
+    if (!y || !m || !d) return iso || "";
+    return new Date(y, m - 1, d).toLocaleDateString(undefined, {
+      year: "numeric", month: "short", day: "numeric",
+    });
+  }
+
+  document.getElementById("changelog-btn").addEventListener("click", async () => {
+    let entries = [];
+    try {
+      const res = await fetch("changelog.json");
+      if (res.ok) {
+        const body = await res.json();
+        if (Array.isArray(body)) entries = body.filter(e => e && e.text);
+      }
+    } catch { /* fall through to empty state */ }
+
+    changelogList.innerHTML = "";
+    changelogEmpty.hidden = entries.length > 0;
+    let lastDate = null;
+    for (const e of entries) {
+      if (e.date !== lastDate) {
+        lastDate = e.date;
+        const dt = document.createElement("li");
+        dt.className = "changelog-date";
+        dt.textContent = formatChangelogDate(e.date);
+        changelogList.appendChild(dt);
+      }
+      const li = document.createElement("li");
+      li.className = "changelog-entry";
+      li.textContent = e.text;
+      changelogList.appendChild(li);
+    }
+    changelogDialog.showModal();
+  });
+
   // ---------- exercise dialog (add / edit a training exercise) ----------
   const exerciseDialog = document.getElementById("exercise-dialog");
   const exerciseForm   = document.getElementById("exercise-form");