fix: stable y-axis range and sane dist_km in activity charts

Three bugs in the time↔distance x-axis toggle:

1. GPS speed glitches (e.g. a 1-second spike at 222 km/h) were accumulated
   into dist_km, pushing all subsequent points ~60 m too far right on the
   distance axis and compressing the rest of the chart.  Cap speed at 150 km/h
   during dist_km integration; values above that are treated as 0 movement.

2. Observable Plot auto-infers the y domain from plottable points only.
   When x-mode changes, which points are "plottable" changes too, so the
   y axis range silently shifted between time and distance views.  Fix by
   computing lineDomainMin/Max once from the full dataset and passing an
   explicit domain to Plot.

3. monotone-x curve requires strictly increasing x.  In distance mode,
   stopped segments produce consecutive points with identical dist_km,
   causing NaN Bézier control points and visual artifacts.  Use linear
   curve for distance mode (data is dense enough that it looks smooth).

site/src/components/ActivityCharts.svelte

@@ -21,15 +21,20 @@
   let chartEl: HTMLDivElement;
   let chart: SVGElement | null = null;
 
-  // Cumulative distance in km, integrated from speed_kmh
+  // Cumulative distance in km, integrated from speed_kmh.
+  // Speeds > 150 km/h are treated as 0 (GPS glitch guard) — otherwise a single
+  // 1-second spike at 220 km/h pushes all subsequent points ~60 m too far right
+  // on the distance axis and stretches the chart out of proportion.
   $: dist_km = (() => {
     if (!timeseries.speed_kmh.some(v => v != null)) return null;
-    const d: (number | null)[] = [0];
+    const d: number[] = [0];
     for (let i = 1; i < timeseries.t.length; i++) {
       const v = timeseries.speed_kmh[i];
       const dt = timeseries.t[i] - timeseries.t[i - 1];
       const prev = d[i - 1];
-      d.push(v != null && prev != null ? prev + v * dt / 3600 : prev);
+      // Clamp to 150 km/h; treat null or out-of-range as 0 movement
+      const vSafe = (v != null && v > 0 && v <= 150) ? v : 0;
+      d.push(prev + vSafe * dt / 3600);
     }
     return d;
   })();
@@ -79,6 +84,15 @@
   $: dataMin = metricValues.length ? Math.min(...metricValues) : 0;
   $: dataMax = metricValues.length ? Math.max(...metricValues) : 100;
 
+  // Explicit y domain for the line chart.
+  // We compute this once from all data and pass it explicitly to Plot so that
+  // switching x-axis mode (time ↔ distance) never changes the y range — Observable
+  // Plot auto-infers different domains when the x-channel changes because it only
+  // considers plottable points, but we want the scale to stay anchored to the
+  // full dataset.  areaY extends down to 0, so include 0 in the minimum.
+  $: lineDomainMin = Math.min(0, dataMin);
+  $: lineDomainMax = dataMax;
+
   // Range handles — reset whenever the metric or chart type changes
   let trimMin = 0;
   let trimMax = 100;
@@ -183,12 +197,18 @@
     const tc = getThemeColors();
     const marks: any[] = [];
 
+    // monotone-x requires strictly increasing x. In time mode t is always
+    // strictly increasing. In distance mode, stopped segments produce many
+    // consecutive points with identical dist_km, which causes NaN Bézier
+    // control points and visual artifacts — use linear instead.
+    const curve = xMode === 'distance' ? 'linear' : 'monotone-x';
+
     if (activeTab === 'cadence') {
-      marks.push(Plot.lineY(data, { x, y: yKey, stroke: color, strokeWidth: 1.5, curve: 'monotone-x' }));
+      marks.push(Plot.lineY(data, { x, y: yKey, stroke: color, strokeWidth: 1.5, curve }));
     } else {
       marks.push(
-        Plot.areaY(data, { x, y: yKey, fill: color, fillOpacity: 0.15, curve: 'monotone-x' }),
+        Plot.areaY(data, { x, y: yKey, fill: color, fillOpacity: 0.15, curve }),
-        Plot.lineY(data, { x, y: yKey, stroke: color, strokeWidth: 1.5, curve: 'monotone-x' }),
+        Plot.lineY(data, { x, y: yKey, stroke: color, strokeWidth: 1.5, curve }),
       );
     }
 
@@ -216,7 +236,7 @@
       width: w, height: h, marginLeft: 48, marginBottom: 32,
       style: { background: 'transparent', color: tc.axis, fontSize: '11px' },
       x: { label: null, tickFormat: xTickFormat, grid: false, ticks: 6 },
-      y: { label: yLabel, grid: true, tickCount: 4 },
+      y: { label: yLabel, grid: true, tickCount: 4, domain: [lineDomainMin, lineDomainMax] },
       marks,
     });
   }