Auto-set app-base meta from BasePath config at startup

Program.cs

@@ -75,6 +75,7 @@ var basePath = builder.Configuration["BasePath"];
 if (!string.IsNullOrWhiteSpace(basePath))
 {
     app.UsePathBase(basePath);
+    UpdateIndexMetaBase(app.Environment, basePath);
 }
 
 app.UseGlobalExceptionLogging();
@@ -100,3 +101,34 @@ app.MapResultsEndpoints();
 app.MapAdminEndpoints();
 
 app.Run();
+
+static void UpdateIndexMetaBase(IWebHostEnvironment env, string basePath)
+{
+    try
+    {
+        var indexPath = Path.Combine(env.WebRootPath, "index.html");
+        if (!File.Exists(indexPath)) return;
+
+        var text = File.ReadAllText(indexPath);
+        var marker = "name=\"app-base\"";
+        var contentKey = "content=\"";
+        var markerIndex = text.IndexOf(marker, StringComparison.OrdinalIgnoreCase);
+        if (markerIndex < 0) return;
+        var contentIndex = text.IndexOf(contentKey, markerIndex, StringComparison.OrdinalIgnoreCase);
+        if (contentIndex < 0) return;
+        var valueStart = contentIndex + contentKey.Length;
+        var valueEnd = text.IndexOf('"', valueStart);
+        if (valueEnd < 0) return;
+
+        var current = text[valueStart..valueEnd];
+        var normalized = basePath.EndsWith('/') ? basePath.TrimEnd('/') : basePath;
+        if (current == normalized) return;
+
+        var updated = text[..valueStart] + normalized + text[valueEnd..];
+        File.WriteAllText(indexPath, updated);
+    }
+    catch
+    {
+        // If we can't rewrite, continue; frontend can still be set manually.
+    }
+}