fix(shared): the tour metrics pass the strict shared typecheck

computeTourMetrics indexed points[i - 1] and points[i], which the shared
package's noUncheckedIndexedAccess reads as possibly undefined, so the
shared typecheck in CI failed since the metrics moved there. It walks the
points pairwise from the first one now; the numbers are the same.

shared/src/tours/tours.metrics.ts

@@ -23,7 +23,8 @@ export interface TourMetrics {
  * ones for a tour saved offline, so both read them from here.
  */
 export function computeTourMetrics(points: readonly GeometryPoint[]): TourMetrics {
-  if (points.length < 2) {
+  const [first, ...rest] = points;
+  if (!first || rest.length === 0) {
     return { distanceKm: 0, elevationGainM: null, elevationLossM: null, hasElevation: false };
   }
   const hasElevation = points.every((p) => p.length === 3 && Number.isFinite(p[2]));
@@ -31,13 +32,15 @@ export function computeTourMetrics(points: readonly GeometryPoint[]): TourMetric
   let distanceKm = 0;
   let gain = 0;
   let loss = 0;
-  for (let i = 1; i < points.length; i++) {
+  let prev = first;
-    distanceKm += haversineMetres(points[i - 1][0], points[i - 1][1], points[i][0], points[i][1]) / 1000;
+  for (const point of rest) {
+    distanceKm += haversineMetres(prev[0], prev[1], point[0], point[1]) / 1000;
     if (hasElevation) {
-      const delta = (points[i][2] as number) - (points[i - 1][2] as number);
+      const delta = (point[2] as number) - (prev[2] as number);
       if (delta > 0) gain += delta;
       else loss += -delta;
     }
+    prev = point;
   }
 
   return {