feat(hikes): medium hero variant + Switzerland-framed overview, drop static→live wobble

Three related improvements to the pre-rendered hero map system:

* New medium viewport variant (561–899 CSS px) for the per-hike detail
  hero and the /hikes overview. Tablet/split-pane viewports were
  getting the wide pose (chosen for ~1920 CSS px), which landed too
  zoomed in. Each variant is rendered at a pose matching its
  container, so the static→Leaflet handover aligns at every band.
  Manifest fields are optional — pages fall back to the wide variant
  on tablets until build-hikes regenerates the images.

* Overview frames on Switzerland (fixed center [46.82, 8.23]) with
  explicit per-variant zooms (wide=8, medium=8, narrow=7) rather than
  auto-fitting the union of hike bboxes. The previous behavior zoomed
  in on whichever corner the catalogue clustered in; this reads as
  "hikes across CH". Bumps OVERVIEW_RENDER_VERSION so cached overview
  images get invalidated on the next build.

* Removed the post-tile-load flyToBounds in both HikeMap.svelte and
  HikesOverviewMap.svelte. The map already opens at the static pose
  via setView; the second auto-fit was adding a visible wobble on
  routes whose bbox sits at an integer-zoom boundary (e.g. the
  Einsiedeln–Unteriberg detail), where the build-time fit and
  Leaflet's runtime fit disagree by one zoom step at the user's
  actual container size.

scripts/staticHikeMap.ts

@@ -30,7 +30,7 @@ function tileUrl(sub: string, layer: string, z: number, x: number, y: number): s
 }
 
 /** Web Mercator: lng/lat → absolute pixel coordinate at a given zoom. */
-function lngLatToPx(lng: number, lat: number, zoom: number): { x: number; y: number } {
+export function lngLatToPx(lng: number, lat: number, zoom: number): { x: number; y: number } {
 	const n = 2 ** zoom;
 	const x = ((lng + 180) / 360) * n * TILE_SIZE;
 	const latRad = (lat * Math.PI) / 180;