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fix(hikes): repair Swisstopo elevation API (LV95 + POST), add busy chip

Two upstream constraints were causing the densified route-builder
tracks to ship without `<ele>` on most points:

1. api3.geo.admin.ch's elevation services reject `sr=4326` outright
   (HTTP 400: "Please provide a valid number for the spatial
   reference system model: 21781, 2056"). Add an in-process
   WGS84→LV95 converter using Swisstopo's published approximation
   (≈1 m positional accuracy, well below the DTM grid resolution)
   and switch both `height` and `profile.json` to sr=2056.

2. profile.json GET silently 414s once the URL crosses ~8 KB. At
   our densified-track sizes a 200-coord chunk hit ~9.6 KB and got
   dropped — that's why only a handful of segments came back with
   elevations. Switch to POST + form-encoded body; chunk size can
   safely go to 500 coords.

UX: extend the busy state to cover the densify+elevate path
(previously only set during snap-to-route) and add a mode-agnostic
status chip in the header that pulses amber while the elevation
request is in flight. GPX download button is now disabled while
busy so the file can't be exported half-finished.

2026-05-19 11:01:23 +02:00