fix elevation_gain_m null for modern Garmin FIT files; fix map flash

FIT parser: try enhanced_altitude before altitude. Barometric altimeters
on modern Garmins (Edge 540, 840, etc.) write enhanced_altitude in
record messages and total_ascent in lap messages. The old code read only
altitude, producing null elevation_m per point → null elevation_gain_m
at the activity root while laps had correct values from total_ascent.

ActivityMap: use preview_coords (passed from ActivitySummary) to
initialise the map at the activity's location on mount, eliminating the
flash of world-view before the async detail JSON / bbox arrives.

site/src/components/ActivityDetail.svelte

@@ -244,6 +244,7 @@
         {trackUrl}
         {timeseries}
         bbox={detail?.bbox ?? null}
+        initialCoords={activity.preview_coords}
         accentColor={color}
         bind:hoveredIdx
       />

site/src/components/ActivityMap.svelte

@@ -7,6 +7,10 @@
   export let trackUrl: string;
   export let timeseries: Timeseries | null = null;
   export let bbox: [number, number, number, number] | null = null;
+  /** ~20 [lat, lon] preview coords from the activity summary — used to position
+   *  the map immediately on mount so there's no flash of world view before the
+   *  detail JSON loads and bbox arrives. */
+  export let initialCoords: [number, number][] | null = null;
   export let accentColor: string = '#00c8ff';
   export let hoveredIdx: number | null = null;
 
@@ -19,11 +23,25 @@
   const TILE_STYLE = 'https://tiles.openfreemap.org/styles/positron';
 
   onMount(() => {
+    // Derive initial center and zoom from preview_coords so the map starts at
+    // the right location without waiting for the async detail JSON / bbox load.
+    let initCenter: [number, number] = [0, 0];
+    let initZoom = 1;
+    if (initialCoords && initialCoords.length > 0) {
+      const lats = initialCoords.map(c => c[0]);
+      const lons = initialCoords.map(c => c[1]);
+      initCenter = [
+        (Math.min(...lons) + Math.max(...lons)) / 2,
+        (Math.min(...lats) + Math.max(...lats)) / 2,
+      ];
+      initZoom = 10; // rough default; fitBounds will correct this when bbox arrives
+    }
+
     map = new maplibregl.Map({
       container: mapEl,
       style: TILE_STYLE,
-      center: [0, 0],
-      zoom: 1,
+      center: initCenter,
+      zoom: initZoom,
       attributionControl: false,
     });