feat(route-builder): import existing GPX (round-trip editing)

Lets the user re-load a previously-exported GPX and keep iterating
on the same route — add a waypoint, fix a turn, retag an image —
without rebuilding from scratch.

The exported GPX interleaves user-anchor waypoints with densified /
snapped intermediates in a single `<trkseg>`. The importer doesn't
try to perfectly round-trip "manual waypoint vs intermediate";
instead it recovers the *image* anchors by matching `<wpt>`
coordinates against the trkpt sequence (1e-5° tolerance, ≈1 m),
plus the start + end trkpts, and reconstructs routedSegments from
the trkpts between adjacent anchors. The intermediate geometry is
preserved verbatim — no re-routing, no second elevation pass.

Image waypoints carry their `imageHash` + `imageVisibility` across
the round-trip so the build script can still re-attach the source
JPEGs on the next publish. Visual previews from those hashes are
deferred to a follow-up — for now an image anchor renders as a
hash-only badge in the waypoint table.

Auto-snap is forced off after import so the freshly-loaded geometry
isn't immediately overwritten by a routing API call. UI: a "GPX
laden" link-style button next to the existing Reset, confirms
before replacing a non-empty draft.

The pure parsers (`parseGpx`, `parseGpxImageRefs`) move from
`$lib/server/gpx` to `$lib/gpx` so the browser-side importer can
use them; the server module re-exports for back-compat.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "homepage",
-	"version": "1.76.1",
+	"version": "1.77.0",
 	"private": true,
 	"type": "module",
 	"scripts": {