Format every metre/kilometre distance through one shared helper

Directions, the Contacts marker and panel, placed annotation labels, the
drawn-line measure, the Cockpit and formatRouteDistance each chose the unit
and precision on their own, and most chose before rounding: #1450, #1433 and
formatDistanceText or kilometreDigits from src/data/distanceFormat.js, which
cut after rounding. Each keeps its own metre step and rendering.

That also fixes the 10 km cut the Contacts marker and panel, the drawn-line
measure and the Cockpit still had (9,960 m read 10.0 km). The helper returns
kilometres already rounded, so the plain marker and the localized panel no
longer disagree on half-tenths such as 1,450 m.

A negative annotation distance now leaves the label without a distance, and
a drawn line with a coordinate that is not a number shows no measure, rather
than printing 0 m or NaN km. The HUD altitude tag and the spoken view height
keep their own unit choice.

src/data/routeSteps.js

@@ -13,6 +13,8 @@
  * roundabout turn, exit roundabout, exit rotary, notification, use lane.
  */
 
+import { formatDistanceText } from './distanceFormat.js';
+
 /** Cap on steps returned for one route — a 2,500 km drive rarely needs more. */
 export const ROUTE_STEPS_MAX = 200;
 
@@ -216,11 +218,7 @@ export function normalizeOsrmSteps(route) {
  * @returns {string}
  */
 export function formatRouteDistance(meters) {
-  if (!Number.isFinite(meters) || meters < 0) return '';
-  if (meters < 100) return `${Math.round(meters)} m`;
-  if (meters < 1000) return `${Math.round(meters / 10) * 10} m`;
-  const km = meters / 1000;
-  return `${km < 10 ? km.toFixed(1) : Math.round(km)} km`;
+  return formatDistanceText(meters, { metreStep: 10, exactBelowM: 100 }) ?? '';
 }
 
 /**