fix: flip CVSR train icon based on GPS heading (#532)

[fatherlinux]

Summary

• Train icon now faces the direction of travel instead of always following the track linestring digitization direction
• Compares the snap-to-line bearing against the GPS heading from the USFT tracker
• If they differ by more than 90°, flips the bearing by 180°
• 3-line change in the snappedTrain useMemo in Map.jsx

Closes #532

Test plan

• Verify train icon faces north when train is heading northbound
• Verify train icon faces south when train is heading southbound
• Verify icon still snaps to track geometry (no floating off the line)
• Verify smooth rotation transition when direction changes

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates the snapped train's bearing calculation in the Map component to align the train icon with its actual direction of travel based on the GPS heading. The feedback suggests only performing this bearing adjustment when a valid numeric GPS heading is present to prevent the icon from incorrectly flipping or facing North when heading data is unavailable.

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[gemini-code-assist]

Defaulting to 0 when trainPosition.heading is null or undefined can cause the train icon to incorrectly flip or face North (0°) when heading data is unavailable. We should only perform the bearing adjustment if a valid numeric GPS heading is present.

    if (typeof trainPosition.heading === 'number') {
      const gpsHeading = trainPosition.heading;
      const diff = Math.abs(((snap.bearing - gpsHeading + 540) % 360) - 180);
      if (diff > 90) snap.bearing = (snap.bearing + 180) % 360;
    }
    return snap;