Add a Distance/Time toggle to the profile chart's x-axis

TrackStats.build_profile now also computes elapsed_secs per point
(relative to the first known timestamp), alongside the existing
cumulative distance. Point x-positions stay distance-based either
way — only the shared axis strip's 5 tick labels (and the tooltip)
switch between the two, so toggling is a cheap re-render rather than
rebuilding the charts. The toggle only appears when a track actually
has elapsed-time data to switch to.

public/css/style.css

@@ -619,6 +619,52 @@ html, body {
   bottom: 2px;
 }
 
+.profile-panel__xaxis-toggle {
+  display: flex;
+  justify-content: flex-end;
+  gap: 4px;
+  margin-bottom: 4px;
+}
+
+.profile-panel__xaxis-toggle-btn {
+  border: 1px solid #e1e0d9;
+  background: #fcfcfb;
+  color: #52514e;
+  font-size: 11px;
+  padding: 2px 8px;
+  border-radius: 3px;
+  cursor: pointer;
+}
+
+.profile-panel__xaxis-toggle-btn.active {
+  background: #0b0b0b;
+  border-color: #0b0b0b;
+  color: #fff;
+}
+
+.profile-panel__xaxis {
+  position: relative;
+  height: 14px;
+  margin-top: 6px;
+}
+
+.profile-panel__xaxis-tick {
+  position: absolute;
+  top: 0;
+  transform: translateX(-50%);
+  font-size: 10px;
+  color: #898781;
+  white-space: nowrap;
+}
+
+.profile-panel__xaxis-tick:first-child {
+  transform: translateX(0);
+}
+
+.profile-panel__xaxis-tick:last-child {
+  transform: translateX(-100%);
+}
+
 .profile-panel__tooltip {
   position: absolute;
   top: 0;