fix: re-georeference flood zones and make them clearly visible

The previous affine fit slid along the mostly north-south river (an
ill-conditioned direction) leaving the zones ~1 km south-west of their
true position. New approach: extract the scanned river band by color,
match it to the OSM Ping mainstem centerline by arc length (which pins
the along-river position), then correct residual translation using
known district locations cross-checked against the river residual
(533 m -> ~300 m median; Chang Khlan zone centroid now within 200 m).
Mountain-area artifacts are clipped away via the municipality hull.

Zones were also nearly invisible at fillOpacity 0.16 - base opacity is
now 0.38, rising with the live 24 h exceedance probability, and 0.72
with a solid red border once the river is at or above a zone's level.

src/static/dashboard.html

@@ -624,10 +624,10 @@
         const flooded = state.p1Now != null && state.p1Now >= level;
         return {
             color: flooded ? '#7a1512' : '#8a5a2b',
-            weight: flooded ? 2.5 : 1,
+            weight: flooded ? 2.5 : 1.2,
             dashArray: flooded ? null : '4 4',
             fillColor: ZONE_COLORS[zone] || '#f9e3a1',
-            fillOpacity: flooded ? .55 : .16 + .38 * Math.min(1, p),
+            fillOpacity: flooded ? .72 : .38 + .3 * Math.min(1, p),
         };
     }