fix: replace OSM tiles with ArcGIS for China network compatibility

- Switch primary tile provider from OpenStreetMap (blocked in China)
  to ArcGIS World Street Map (certified working)
- Add fallback chain: ArcGIS -> ArcGIS Gray -> OSM
- Auto-detect tile failures and fall back to next provider
- Clean up init sequence for reliable map rendering

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

simple_analysis/templates/simple_analysis/simple_analysis.html

@@ -664,10 +664,29 @@ function renderClusterCards(clusters) {
 }
 
 /* ── Step 4: Map & Visualization ──────────────────────── */
+
+// Tile providers in priority order — ArcGIS works in China
+const TILE_PROVIDERS = [
+    {
+        name: 'ArcGIS',
+        url: 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}',
+        attr: 'Tiles © Esri — Source: Esri, DeLorme, NAVTEQ'
+    },
+    {
+        name: 'ArcGIS Gray',
+        url: 'https://server.arcgisonline.com/ArcGIS/rest/services/Canvas/World_Light_Gray_Base/MapServer/tile/{z}/{y}/{x}',
+        attr: 'Tiles © Esri — Esri, DeLorme, NAVTEQ'
+    },
+    {
+        name: 'OSM',
+        url: 'https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png',
+        attr: '© OpenStreetMap contributors'
+    },
+];
+
 function initMap() {
     const container = document.getElementById('saMap');
     if (SA.map) {
-        // Ensure container is visible and properly sized
         container.style.height = '550px';
         setTimeout(() => {
             SA.map.invalidateSize();
@@ -675,7 +694,6 @@ function initMap() {
         }, 200);
         return;
     }
-    // Wait for container to be visible, then init map
     setTimeout(() => {
         container.style.height = '550px';
         // Start with global view [0, 0] zoom 2 as required
@@ -684,15 +702,35 @@ function initMap() {
             zoom: 2,
             worldCopyJump: true,
         });
-        L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
-            attribution: '© OpenStreetMap contributors',
-            maxZoom: 18,
-        }).addTo(SA.map);
+        // Try each tile provider in order until one loads
+        tryTileProvider(0);
         setTimeout(() => SA.map.invalidateSize(), 300);
         renderMapData();
     }, 250);
 }
 
+function tryTileProvider(idx) {
+    if (idx >= TILE_PROVIDERS.length || !SA.map) return;
+    const provider = TILE_PROVIDERS[idx];
+    const tileLayer = L.tileLayer(provider.url, {
+        attribution: provider.attr,
+        maxZoom: 18,
+    });
+    tileLayer.addTo(SA.map);
+
+    // Detect if tiles fail to load, try next provider
+    let tileError = false;
+    tileLayer.on('tileerror', () => {
+        if (!tileError) {
+            tileError = true;
+            console.warn('Tile provider failed:', provider.name);
+            SA.map.removeLayer(tileLayer);
+            tryTileProvider(idx + 1);
+        }
+    });
+    SA._tileLayer = tileLayer;
+}
+
 function renderMapData() {
     if (!SA.map) return;