Topography visual update
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This version keeps the continuous legend model and improves only the raster/legend appearance:

1. Percentile contrast stretch
   - Colors are normalized with the 2nd and 98th percentiles.
   - True min/max are still kept for statistics and the legend top/bottom labels.

2. 256-color continuous ramp
   - The backend samples the WRAP-style color ramp into 256 interpolated RGB colors.
   - Pixels are not classified into elevation breaks.

3. Light Gaussian smoothing
   - A small mask-aware Gaussian smoothing is applied to the normalized elevation surface before coloring.
   - The smoothing uses normalized convolution so NoData outside the watershed does not bleed into the watershed edge.

4. Cleaner continuous legend
   - The middle label was removed.
   - The legend shows only maximum and minimum labels plus the vertical gradient bar.
