fix: 使用L.svg渲染器避免preferCanvas冲突

Leaflet map使用preferCanvas:true时,所有polyline经canvas渲染,
dashOffset动画可能不生效。为流量层显式指定L.svg()渲染器。

Co-Authored-By: Claude <noreply@anthropic.com>

simple_analysis/templates/simple_analysis/simple_analysis.html

@@ -2598,7 +2598,8 @@ function toggleTrafficPanel() {
 function _ensureFlowLayer() {
     if (!SA.map) return null;
     if (!_trafficFlowLayer) {
-        _trafficFlowLayer = L.layerGroup().addTo(SA.map);
+        const svgRenderer = L.svg({padding: 0.5});
+        _trafficFlowLayer = L.layerGroup([], {renderer: svgRenderer}).addTo(SA.map);
     }
     return _trafficFlowLayer;
 }