Activity map: color track by speed when hovering speed stats

Hovering avg/max speed in the stats panel switches the track from the
default gradient to a blue→green→yellow→red speed gradient. Progress
along the track is computed from cumulative distance (speed × time) so
fast sections appear proportionally long rather than time-weighted.
Reverts to default on mouse-leave. No-op when timeseries has no speed
data or the activity has no GPS track.

site/src/components/ActivityMap.svelte

@@ -13,15 +13,92 @@
   export let initialCoords: [number, number][] | null = null;
   export let accentColor: string = '#00c8ff';
   export let hoveredIdx: number | null = null;
+  /** When 'speed', colors the track by speed using a blue→green→yellow→red gradient. */
+  export let colorMode: 'default' | 'speed' = 'default';
 
   let mapEl: HTMLDivElement;
   let map: any;
   const MarkerClass = maplibregl.Marker;
   let hoverMarker: any;
   let markersAdded = false;
+  let mapLoaded = false;
 
   const TILE_STYLE = 'https://tiles.openfreemap.org/styles/positron';
 
+  const defaultGradient = [
+    'interpolate', ['linear'], ['line-progress'],
+    0,   accentColor,
+    0.5, '#ff6b35',
+    1,   accentColor,
+  ];
+
+  // Blue → green → yellow → red scale, t ∈ [0, 1]
+  function _speedColor(t: number): string {
+    const stops: [number, [number, number, number]][] = [
+      [0,    [59,  130, 246]],  // blue-500
+      [0.33, [74,  222, 128]],  // green-400
+      [0.66, [250, 204, 21 ]],  // yellow-400
+      [1,    [239, 68,  68 ]],  // red-500
+    ];
+    for (let i = 0; i < stops.length - 1; i++) {
+      const [t0, c0] = stops[i], [t1, c1] = stops[i + 1];
+      if (t >= t0 && t <= t1) {
+        const f = (t - t0) / (t1 - t0);
+        return `rgb(${Math.round(c0[0] + (c1[0] - c0[0]) * f)},${Math.round(c0[1] + (c1[1] - c0[1]) * f)},${Math.round(c0[2] + (c1[2] - c0[2]) * f)})`;
+      }
+    }
+    return '#60a5fa';
+  }
+
+  function buildSpeedGradient(ts: Timeseries): any[] | null {
+    const t = ts.t, speeds = ts.speed_kmh;
+    if (!t?.length || !speeds?.length) return null;
+    const n = Math.min(t.length, speeds.length);
+
+    // Compute cumulative distance via speed integration so fast sections
+    // map to proportionally longer segments on the track (vs time-based).
+    const cumDist: number[] = [0];
+    let lastSpd = 0;
+    for (let i = 1; i < n; i++) {
+      const spd = speeds[i] != null ? speeds[i]! : lastSpd;
+      if (speeds[i] != null) lastSpd = spd;
+      cumDist.push(cumDist[i - 1] + (spd / 3.6) * (t[i] - t[i - 1]));
+    }
+    const total = cumDist[n - 1];
+    if (!total) return null;
+
+    const valid = speeds.filter((s): s is number => s != null);
+    if (!valid.length) return null;
+    const minSpd = Math.min(...valid);
+    const maxSpd = Math.max(...valid);
+    const range = maxSpd - minSpd;
+
+    const expr: any[] = ['interpolate', ['linear'], ['line-progress']];
+    let prev = -1;
+    let lastFill = minSpd;
+    for (let i = 0; i < n; i++) {
+      const progress = Math.min(cumDist[i] / total, 1);
+      if (progress <= prev + 0.001) continue;
+      prev = progress;
+      const spd = speeds[i] != null ? speeds[i]! : lastFill;
+      if (speeds[i] != null) lastFill = spd;
+      expr.push(progress, _speedColor(range > 0 ? (spd - minSpd) / range : 0.5));
+    }
+    if (prev < 1) expr.push(1, expr[expr.length - 1]); // close at 1.0
+    return expr.length >= 6 ? expr : null; // need ≥ 2 stops
+  }
+
+  function _applyGradient() {
+    if (!map?.getLayer('track-line')) return;
+    const gradient = colorMode === 'speed' && timeseries
+      ? (buildSpeedGradient(timeseries) ?? defaultGradient)
+      : defaultGradient;
+    map.setPaintProperty('track-line', 'line-gradient', gradient);
+  }
+
+  // Re-apply whenever colorMode or timeseries changes (once map is loaded).
+  $: if (mapLoaded) { colorMode; timeseries; _applyGradient(); }
+
   onMount(() => {
     // Derive initial center and zoom from preview_coords so the map starts at
     // the right location without waiting for the async detail JSON / bbox load.
@@ -59,6 +136,7 @@
       .addTo(map);
 
     map.on('load', () => {
+      mapLoaded = true;
       map.addSource('track', {
         type: 'geojson',
         data: trackUrl,