ActivityCharts: switch slope tab to distance-weighted smoothing

Replace time-based rolling mean with a 400 m distance-weighted sliding-
window average (O(n), two advancing pointers) matching the spec in
SLOPE_COLORING.md. Slope values are now spatially consistent regardless
of riding speed. Smoothing buttons are hidden when the slope tab is
active.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

site/src/components/ActivityCharts.svelte

@@ -151,21 +151,33 @@
     return '#a855f7';                // wall
   }
 
+  // Distance-weighted sliding-window slope smoothing (400 m window).
+  // O(n) via two advancing pointers. Matches SLOPE_COLORING.md spec.
+  function computeSmoothedSlopes(pts: { d: number; e: number }[], windowM = 400): number[] {
+    if (pts.length < 2) return pts.map(() => 0);
+    const half = windowM / 2;
+    const segs: { d: number; slope: number; len: number }[] = [];
+    for (let i = 0; i < pts.length - 1; i++) {
+      const len = pts[i + 1].d - pts[i].d;
+      segs.push({ d: (pts[i].d + pts[i + 1].d) / 2, slope: len > 0.5 ? (pts[i + 1].e - pts[i].e) / len * 100 : 0, len });
+    }
+    const slopes = new Array<number>(pts.length);
+    let lo = 0, hi = 0, sumW = 0, sumS = 0;
+    for (let i = 0; i < pts.length; i++) {
+      const center = pts[i].d;
+      while (hi < segs.length && segs[hi].d <= center + half) { sumW += segs[hi].len; sumS += segs[hi].slope * segs[hi].len; hi++; }
+      while (lo < hi && segs[lo].d < center - half)           { sumW -= segs[lo].len; sumS -= segs[lo].slope * segs[lo].len; lo++; }
+      slopes[i] = sumW > 0.5 ? sumS / sumW : 0;
+    }
+    return slopes;
+  }
+
   $: slopeData = (() => {
     if (!dist_km) return null;
-    const d = dist_km;
+    const raw = timeseries.elevation_m;
-    const halfWin = SMOOTH_HALF[smoothMode];
+    const pts = dist_km.map((d, i) => ({ d: d * 1000, e: raw[i] ?? 0 }));
-    const elev = halfWin > 0 ? rollingMean(timeseries.elevation_m, halfWin) : timeseries.elevation_m;
+    const slopes = computeSmoothedSlopes(pts);
-    return data.map((pt, i) => {
+    return data.map((pt, i) => ({ ...pt, slope: slopes[i] }));
-      const e0 = elev[i] ?? 0;
-      let slope = 0;
-      if (i < d.length - 1) {
-        const dDist = (d[i + 1] - d[i]) * 1000;
-        const e1 = elev[i + 1] ?? e0;
-        if (dDist > 0.5) slope = ((e1 - e0) / dDist) * 100;
-      }
-      return { ...pt, elevation: elev[i], slope };
-    });
   })();
 
   function injectSlopeGradient(svg: SVGElement, pts: { dist_km: number | null; slope: number }[], w: number) {
@@ -526,7 +538,7 @@
       </div>
     {/if}
 
-    {#if chartType === 'line'}
+    {#if chartType === 'line' && activeTab !== 'slope'}
       <div class="flex items-center gap-1">
         {#each (['raw', '10s', '20s'] as SmoothMode[]) as sm}
           <button