23-Jan-2026 — cache start coordinates, lazy load GPX list, and refactor ref: #133

build 434 — commit 1c591cb

• Add startLat/startLng fields to FileNameData and ServerFileNameData
• Extract and cache GPX start coordinates at upload time
• Use cached coordinates in list endpoint, only parsing from S3 if needed
• Process GPX files sequentially instead of Promise.all to prevent memory exhaustion
• Auto-cache coordinates on first access for existing files
• Lazy load GPX file list only when dropdown is opened (not on page load)
• Show existing GPX file name without fetching entire list
• Refactor server-side processing into smaller functions