fe08e06a02
Mirrors the per-hike detail-page hero on the /hikes index. Build emits one
WebP at the union bbox of every visible hike with each preview polyline
drawn in its SAC-tier colour; page renders it under the live Leaflet map
and fades it out once the first tile batch loads.
Tile fetcher now distinguishes HTTP 4xx ("intentionally blank — outside
Switzerland") from real network errors, so the larger overview canvas
that extends into DE/IT/FR doesn't trip the network-failure abort.