fix(gpx): add timestamps to all trkpt elements for Strava/Garmin compatibility

[jasoneplumb]

Problem

The GPX export omits <time> elements from <trkpt> nodes in some or all segments. Most apps (Strava, Garmin Connect, komoot) require <trkpt time="..."> on every point to:

• Build elevation-over-time graphs
• Calculate moving time vs. elapsed time
• Merge splits correctly across pause segments

Without timestamps, imports either fail silently or strip the track to a static polyline with no time data.

Current State

Check src/recording.ts — the trailPoints array stores {latlng, t, speedMs, altM?} where t is a performance.now() timestamp. The GPX writer converts these but may not be attaching <time> to every point, or may be using relative timestamps instead of absolute ISO 8601.

Fix

Each <trkpt> must include:

<trkpt lat="..." lon="...">
  <ele>...</ele>
  <time>2026-04-18T14:23:01.000Z</time>
</trkpt>

t from performance.now() is milliseconds since page load — convert to absolute wall-clock time using Date.now() - performance.now() + t at the time of GPX export.

Files

• src/recording.ts — GPX export function (search for gpx or trkpt)

Suggested Prompt

Fix GPX export to include ISO 8601 timestamps on every trkpt element (issue #NNN).
In src/recording.ts, find the GPX builder. Each trail point has a t field (performance.now() ms). Convert to absolute UTC using const epochOffset = Date.now() - performance.now() captured at export time, then new Date(epochOffset + point.t).toISOString(). Add a <time> child to every <trkpt>. Verify the output is valid GPX 1.1 (schema at topografix.com/GPX/1/1). Run npm run type-check && npm run lint before reporting done.