Fix follow-ups: lap bests, segments, charts, dashboard health

- Lap bests: compare against OTHER activities on the route (exclude self),
  so single-activity routes no longer show every lap as "best"
- Segment create: POST to trailing-slash URL (was a 307 that dropped the body);
  surface errors in the UI
- PR splits: scale GPS distance stream to the activity's official distance so
  over-measured GPS no longer yields bogus split PRs
- Speed route colours: red->orange->green->blue->purple (slow->fast) with smooth
  interpolation + a Slow/Fast gradient key under the map
- Health body battery: snap activity highlight to the categorical axis;
  white tooltip text + % suffix
- Health weight: y-min = lowest weight - 20kg; st/lb hover shows total lbs too
- Health sleep: move 8h/avg reference labels into the right margin
- Dashboard: Health-today pulls latest non-null values (sleep score, VO2 max);
  body battery tile renders a condensed colour-graded intraday graph

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/activity/ActivityMap.jsx

@@ -28,13 +28,27 @@ const TILE_LAYERS = {
 // buffer of tiles and don't defer loads until the map is idle.
 const TILE_OPTS = { maxZoom: 19, keepBuffer: 6, updateWhenIdle: false, updateWhenZooming: false }
 
-// Slow → fast colour ramp for speed-coloured routes.
-const SPEED_STOPS = ['#3b82f6', '#22c55e', '#eab308', '#f97316', '#ef4444']
+// Slow → fast colour ramp for speed-coloured routes (red → purple).
+export const SPEED_STOPS = ['#ef4444', '#f97316', '#22c55e', '#3b82f6', '#a855f7']
 
-function speedColorIndex(speed, min, max) {
-  if (!(max > min)) return 1
-  const t = (speed - min) / (max - min)
-  return Math.min(SPEED_STOPS.length - 1, Math.max(0, Math.floor(t * SPEED_STOPS.length)))
+// CSS gradient string for the speed legend.
+export const SPEED_GRADIENT = `linear-gradient(to right, ${SPEED_STOPS.join(', ')})`
+
+const SPEED_LEVELS = 24   // quantisation steps → smooth gradient while limiting layer count
+
+function lerpColor(c1, c2, t) {
+  const a = parseInt(c1.slice(1), 16), b = parseInt(c2.slice(1), 16)
+  const r = Math.round(((a >> 16) & 255) + (((b >> 16) & 255) - ((a >> 16) & 255)) * t)
+  const g = Math.round(((a >> 8) & 255) + (((b >> 8) & 255) - ((a >> 8) & 255)) * t)
+  const bl = Math.round((a & 255) + ((b & 255) - (a & 255)) * t)
+  return `#${((1 << 24) + (r << 16) + (g << 8) + bl).toString(16).slice(1)}`
+}
+
+function rampColor(t) {
+  t = Math.max(0, Math.min(1, t))
+  const seg = t * (SPEED_STOPS.length - 1)
+  const i = Math.min(SPEED_STOPS.length - 2, Math.floor(seg))
+  return lerpColor(SPEED_STOPS[i], SPEED_STOPS[i + 1], seg - i)
 }
 
 function decodePolyline(encoded) {
@@ -77,21 +91,26 @@ function drawRoute(map, { polyline, dataPoints, sportType, colorMode }, trackRef
     const lo = speeds[Math.floor(speeds.length * 0.05)] ?? 0
     const hi = speeds[Math.floor(speeds.length * 0.95)] ?? lo + 1
 
-    // Group consecutive points into runs of the same colour bucket → one polyline per run.
+    const levelOf = (s) => {
+      const t = (hi > lo) ? (((s ?? lo) - lo) / (hi - lo)) : 0.5
+      return Math.round(Math.max(0, Math.min(1, t)) * SPEED_LEVELS)
+    }
+
+    // Group consecutive points into runs of the same colour level → one polyline per run.
     let runStart = 0
-    let runIdx = speedColorIndex(speedPts[0].speed_ms ?? lo, lo, hi)
+    let runLevel = levelOf(speedPts[0].speed_ms)
     const flush = (end) => {
       const coords = speedPts.slice(runStart, end + 1).map(p => [p.latitude, p.longitude])
       if (coords.length >= 2) {
-        L.polyline(coords, { color: SPEED_STOPS[runIdx], weight: 3, opacity: 0.95 }).addTo(group)
+        L.polyline(coords, { color: rampColor(runLevel / SPEED_LEVELS), weight: 3, opacity: 0.95 }).addTo(group)
       }
     }
     for (let i = 1; i < speedPts.length; i++) {
-      const idx = speedColorIndex(speedPts[i].speed_ms ?? lo, lo, hi)
-      if (idx !== runIdx) {
+      const level = levelOf(speedPts[i].speed_ms)
+      if (level !== runLevel) {
         flush(i)            // include current point so runs join up
         runStart = i
-        runIdx = idx
+        runLevel = level
       }
     }
     flush(speedPts.length - 1)