Show min/max y-axis values on each profile chart

Plain HTML overlays, not SVG text: the chart's viewBox scales width
and height independently (preserveAspectRatio="none", for a
responsive width at a fixed pixel height), so text drawn inside that
coordinate space would get stretched by the mismatched factor.

public/css/style.css

@@ -578,9 +578,14 @@ html, body {
   font-weight: normal;
 }
 
+.profile-chart__plot {
+  position: relative;
+}
+
 .profile-chart__svg {
   display: block;
   width: 100%;
+  height: 100%;
   cursor: crosshair;
 }
 
@@ -590,6 +595,30 @@ html, body {
   vector-effect: non-scaling-stroke;
 }
 
+/* Overlaid as plain HTML, not SVG text: the chart's viewBox scales width
+   and height independently (preserveAspectRatio="none", so the line fills
+   a responsive width at a fixed pixel height) — text drawn inside that
+   coordinate space would get stretched by the same mismatched factor. */
+.profile-chart__axis-value {
+  position: absolute;
+  left: 4px;
+  color: #898781;
+  font-size: 10px;
+  line-height: 1;
+  background: rgba(252, 252, 251, 0.75);
+  padding: 1px 3px;
+  border-radius: 2px;
+  pointer-events: none;
+}
+
+.profile-chart__axis-value--max {
+  top: 2px;
+}
+
+.profile-chart__axis-value--min {
+  bottom: 2px;
+}
+
 .profile-panel__tooltip {
   position: absolute;
   top: 0;