frontend: guard the TIFF value colour span against zero/negative

`span = hi - lo || 1` only replaced an exactly-zero span, so Min>Max (or a
cleared Min/Max field coercing to 0 via Number("")) gave a negative span
and rendered the pedestal image with an inverted/collapsed colour scale.
Use `hi - lo > 0 ? hi - lo : 1` so a non-positive span falls back to 1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/TiffImageViewer.tsx

@@ -133,7 +133,7 @@ function TiffImageViewer({ url, variant, height = 460 }: MyProps) {
 
         if (variant === "value") {
             const stops = CMAPS[cmapName] ?? CMAPS.Viridis;
-            const span = hi - lo || 1;
+            const span = hi - lo > 0 ? hi - lo : 1;  // guard zero/negative (e.g. Min>Max or a cleared field)
             for (let i = 0; i < data.length; i++) {
                 const v = data[i];
                 const j = i * 4;