Automatically calculate and display size (area or length) for all assets

New Size Column - Asset register includes selectable "Size" column showing calculated dimensions

Asset Details Display - Asset details screen shows "Size" field positioned

Area Display - Shows calculated area in km² for all polygon/area assets (e.g., "0.045 km²")

Line Display - Shows calculated length in metres for all line assets (e.g., "234 m" or "1234 m")

CSV Export - Size values export correctly when column is selected in CSV downloads (e.g., "0.045 km²", "1234 m", or "N/A")