feat: flooded zones render as water and auto-surface

When P.1's current level reaches a zone's trigger level the zone fills
water-blue (solid border, .62 opacity) instead of the amber risk ramp,
and the zone layer adds itself to the map automatically - no toggle
needed during an actual flood. A manual hide sets a session flag so
auto-show does not fight the user.

src/static/dashboard.html

@@ -627,12 +627,16 @@
         const st = zoneStage(zone);
         const p = st ? st.p_exceed : 0;
         const flooded = state.p1Now != null && state.p1Now >= level;
+        if (flooded) {
+            // river has reached this zone's trigger level: render as water
+            return { color: '#1b5e88', weight: 2, dashArray: null, fillColor: '#2f86c8', fillOpacity: .62 };
+        }
         return {
-            color: flooded ? '#7a1512' : '#8a5a2b',
-            weight: flooded ? 2.5 : 1.2,
-            dashArray: flooded ? null : '4 4',
+            color: '#8a5a2b',
+            weight: 1.2,
+            dashArray: '4 4',
             fillColor: ZONE_COLORS[zone] || '#f9e3a1',
-            fillOpacity: flooded ? .72 : .38 + .3 * Math.min(1, p),
+            fillOpacity: .38 + .3 * Math.min(1, p),
         };
     }
 
@@ -650,15 +654,9 @@
             ${flooded ? '<div class="popup-time" style="color:#8d2f22;font-weight:800">River is at or above this zone’s level now</div>' : ''}</div>`;
     }
 
-    async function toggleFloodZones() {
-        if (!state.map) return;
+    async function showFloodZones(fly) {
+        if (!state.map || zoneLayer) return;
         const button = $('zones-toggle');
-        if (zoneLayer) {
-            state.map.removeLayer(zoneLayer);
-            zoneLayer = null;
-            if (button) button.textContent = 'Show flood zones on map';
-            return;
-        }
         try {
             if (!zoneData) {
                 const response = await fetch('/static/flood-zones-p1.geojson');
@@ -668,16 +666,35 @@
             zoneLayer = L.geoJSON(zoneData, { style: zoneStyle })
                 .bindPopup((layer) => zonePopupHtml(layer.feature))
                 .addTo(state.map);
-            state.map.flyToBounds(zoneLayer.getBounds(), { maxZoom: 13, duration: .8 });
             if (button) button.textContent = 'Hide flood zones';
+            if (fly) {
+                state.map.flyToBounds(zoneLayer.getBounds(), { maxZoom: 13, duration: .8 });
                 document.getElementById('station-map').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
+            }
         } catch (error) {
             if (button) button.textContent = 'Flood zones unavailable';
         }
     }
 
+    function toggleFloodZones() {
+        if (zoneLayer) {
+            state.map.removeLayer(zoneLayer);
+            zoneLayer = null;
+            state.zonesUserHidden = true; // don't auto-show again this session
+            const button = $('zones-toggle');
+            if (button) button.textContent = 'Show flood zones on map';
+        } else {
+            state.zonesUserHidden = false;
+            showFloodZones(true);
+        }
+    }
+
     function restyleFloodZones() {
         if (zoneLayer) zoneLayer.setStyle(zoneStyle);
+        // Flooding in progress: surface the zones even if the layer is off,
+        // unless the user explicitly hid it.
+        const anyFlooded = state.p1Now != null && (state.p1Stages || []).some((s) => state.p1Now >= s.level);
+        if (anyFlooded && !zoneLayer && !state.zonesUserHidden) showFloodZones(false);
     }
 
     function pointInRing(lat, lon, ring) {