SegmentCreate: add elevation profile that zooms to selected portion

Shows a dim area for the visible range around the selection (4% padding)
and a blue overlay for the selected segment, with a light stroke on the
top edge. Both the x-domain and y-domain track the selection, so the
chart zooms in as the handles narrow. Elevation min/max labels overlaid
at top-left and bottom-left.

site/src/components/SegmentCreate.svelte

@@ -18,6 +18,7 @@
 
   // ── Track data ────────────────────────────────────────────────────────────
   let gpsPoints: [number, number][] = [];   // [lat, lon]
+  let elevations: (number | null)[] = [];   // parallel to gpsPoints
   let startIdx = 0;
   let endIdx = 0;
   let loadingTrack = false;
@@ -89,13 +90,19 @@
       if (!ts) { trackError = 'Could not load GPS track.'; loadingTrack = false; return; }
 
       const pts: [number, number][] = [];
-      const lats = ts.lat ?? [], lons = ts.lon ?? [];
+      const elevs: (number | null)[] = [];
+      const lats = ts.lat ?? [], lons = ts.lon ?? [], tsElevs: (number | null)[] = ts.elevation_m ?? [];
       for (let i = 0; i < lats.length; i++) {
-        if (lats[i] != null && lons[i] != null) pts.push([lats[i]!, lons[i]!]);
+        if (lats[i] != null && lons[i] != null) {
+          pts.push([lats[i]!, lons[i]!]);
+          const e = tsElevs[i];
+          elevs.push(typeof e === 'number' ? e : null);
+        }
       }
       if (pts.length < 2) { trackError = 'Not enough GPS points.'; loadingTrack = false; return; }
 
       gpsPoints = pts;
+      elevations = elevs;
       startIdx = 0;
       endIdx = pts.length - 1;
       loadingTrack = false;
@@ -172,6 +179,62 @@
   function onStartInput() { if (startIdx >= endIdx) startIdx = Math.max(0, endIdx - 1); }
   function onEndInput()   { if (endIdx <= startIdx) endIdx = Math.min(maxIdx, startIdx + 1); }
 
+  // ── Elevation chart ───────────────────────────────────────────────────────
+  const ECHART_W = 600;
+  const ECHART_H = 80;
+  const ECHART_PAD = 4;  // px margin at top
+
+  $: chartPad = Math.max(1, Math.floor(maxIdx * 0.04));
+  $: viewLo = Math.max(0, startIdx - chartPad);
+  $: viewHi = Math.min(maxIdx, endIdx + chartPad);
+
+  $: elevStats = (() => {
+    const visible: number[] = [];
+    for (let i = viewLo; i <= viewHi; i++) {
+      const e = elevations[i];
+      if (e != null) visible.push(e);
+    }
+    if (!visible.length) return { min: 0, max: 0, range: 1, hasData: false };
+    let min = visible[0], max = visible[0];
+    for (const v of visible) { if (v < min) min = v; if (v > max) max = v; }
+    return { min, max, range: Math.max(1, max - min), hasData: true };
+  })();
+
+  function buildElevPaths(
+    elevs: (number | null)[],
+    si: number, ei: number,
+    vLo: number, vHi: number,
+    stats: { min: number; max: number; range: number; hasData: boolean },
+  ): { dim: string; sel: string; selLine: string } {
+    if (!stats.hasData || !elevs.length) return { dim: '', sel: '', selLine: '' };
+    const span = Math.max(1, vHi - vLo);
+    const usableH = ECHART_H - ECHART_PAD;
+
+    function area(lo: number, hi: number): { area: string; line: string } {
+      let firstX = -1, lastX = -1;
+      const linePts: string[] = [];
+      for (let i = lo; i <= hi; i++) {
+        const e = elevs[i];
+        if (e == null) continue;
+        const x = ((i - vLo) / span) * ECHART_W;
+        const y = ECHART_PAD + usableH * (1 - (e - stats.min) / stats.range);
+        if (firstX < 0) firstX = x;
+        lastX = x;
+        linePts.push(`L${x.toFixed(1)},${y.toFixed(1)}`);
+      }
+      if (!linePts.length) return { area: '', line: '' };
+      const areaPath = `M${firstX.toFixed(1)},${ECHART_H} ${linePts.join(' ')} L${lastX.toFixed(1)},${ECHART_H} Z`;
+      const linePath = `M${linePts[0].slice(1)} ${linePts.slice(1).join(' ')}`;
+      return { area: areaPath, line: linePath };
+    }
+
+    const dimResult = area(vLo, vHi);
+    const selResult = area(Math.max(si, vLo), Math.min(ei, vHi));
+    return { dim: dimResult.area, sel: selResult.area, selLine: selResult.line };
+  }
+
+  $: elevPaths = buildElevPaths(elevations, startIdx, endIdx, viewLo, viewHi, elevStats);
+
   // ── Save ──────────────────────────────────────────────────────────────────
   async function save() {
     if (!canSave) return;
@@ -271,7 +334,7 @@
       </div>
       <button
         class="text-xs text-zinc-400 hover:text-white transition-colors shrink-0"
-        on:click={() => { selectedActivity = null; gpsPoints = []; map?.remove(); mapReady = false; }}
+        on:click={() => { selectedActivity = null; gpsPoints = []; elevations = []; map?.remove(); mapReady = false; }}
       >Change</button>
     </div>
 
@@ -285,6 +348,23 @@
         <div bind:this={mapEl} class="w-full h-full"></div>
       </div>
 
+      <!-- Elevation profile -->
+      {#if elevPaths.dim}
+        <div class="relative mb-4 rounded-lg overflow-hidden bg-zinc-900 border border-zinc-800" style="height:84px">
+          <svg viewBox="0 0 {ECHART_W} {ECHART_H}" preserveAspectRatio="none" class="w-full h-full">
+            <path d={elevPaths.dim} fill="#3f3f46" opacity="0.5" />
+            <path d={elevPaths.sel} fill="#3b82f6" opacity="0.55" />
+            {#if elevPaths.selLine}
+              <path d={elevPaths.selLine} fill="none" stroke="#93c5fd" stroke-width="1.5" />
+            {/if}
+          </svg>
+          <div class="absolute inset-0 flex flex-col justify-between pointer-events-none px-2 py-1">
+            <span class="text-zinc-500 text-xs leading-none">{Math.round(elevStats.max)}m</span>
+            <span class="text-zinc-500 text-xs leading-none">{Math.round(elevStats.min)}m</span>
+          </div>
+        </div>
+      {/if}
+
       <!-- Dual-range slider -->
       <div class="mb-5">
         <p class="text-xs text-zinc-500 mb-2">Drag the handles to set the segment start and end</p>