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feat(hikes): multi-day stages (separate GPX tracks, stage nav, builder)

Represent a multi-day hike as separate named GPX <trk> elements, one per
stage, while still treating the whole thing as one route on the overview.

GPX & build:
- gpx.ts: parseGpxStages (one stage per <trk>) + multi-track buildGpx.
- build-hikes.ts: per-stage stats with totals summed across stages so the
  overnight gaps (distance, time) and the altitude jump between stages are
  excluded; previewBreaks recorded where stages sit >1 km apart.
- types: HikeStage, manifest `stages?` and `previewBreaks?` (both optional —
  single-stage hikes are unchanged).

Detail page:
- HikeStageNav: a light itinerary-stepper switcher (numbered nodes, active
  glows in the accent) writing a shared stageStore.
- Selecting a stage scopes the metrics, elevation profile (x-window),
  map (highlight + zoom, dim the rest) and photo strip/markers; "Alle
  Etappen" shows the whole route.

Overview: live map and the prerendered static composite both break the
preview line across >1 km inter-stage transfers (previewBreaks).

Route builder:
- Mark any placed waypoint as a stage start (named) from the waypoint list
  or the detail panel; export assembles each stage independently into its
  own <trk>; import re-marks stage boundaries from a multi-track GPX.

2026-05-22 14:14:57 +02:00