Add radar test page with auto-play and connection monitoring

- Add MVC test page at /radar/{suburb}/{state} demonstrating API usage
- Implement auto-play on page load with cookie-based settings
- Add connection monitoring with automatic retry on API downtime
- Show update status, cache state, and next check times
- Fix NextUpdateTime calculation to account for background service intervals
- Add graceful error handling and visual status indicators

Utilities/ResponseBuilder.cs

@@ -7,6 +7,7 @@ public static class ResponseBuilder
     /// <summary>
     /// Creates a RadarResponse from a cache folder path, frames, and metadata
     /// </summary>
+    /// <param name="cacheManagementCheckIntervalMinutes">The interval in minutes that the background cache management service checks for updates. Used to calculate NextUpdateTime when cache is invalid.</param>
     public static RadarResponse CreateRadarResponse(
         string cacheFolderPath, 
         List<RadarFrame> frames,
@@ -15,7 +16,8 @@ public static class ResponseBuilder
         string? state = null,
         bool? cacheIsValid = null,
         DateTime? cacheExpiresAt = null,
-        bool isUpdating = false)
+        bool isUpdating = false,
+        int cacheManagementCheckIntervalMinutes = 5)
     {
         var folderInfo = new DirectoryInfo(cacheFolderPath);
         var lastWriteTime = folderInfo.Exists 
@@ -33,6 +35,35 @@ public static class ResponseBuilder
             }
         }
 
+        // Calculate NextUpdateTime based on cache status:
+        // - If cache is valid: NextUpdateTime = max(CacheExpiresAt, next background service check)
+        //   (Background service checks every N minutes, so update might happen at next check after expiry)
+        // - If cache is invalid and updating: NextUpdateTime = estimated completion time (~2 minutes)
+        // - If cache is invalid and NOT updating: NextUpdateTime = next background service check (based on check interval)
+        DateTime? nextUpdateTime = null;
+        
+        // Calculate next background service check time (rounds up to next check interval)
+        var now = DateTime.UtcNow;
+        var minutesUntilNextCheck = cacheManagementCheckIntervalMinutes - (now.Minute % cacheManagementCheckIntervalMinutes);
+        var nextServiceCheck = now.AddMinutes(minutesUntilNextCheck);
+        
+        if (isUpdating)
+        {
+            // Update in progress - estimate completion in ~2 minutes
+            nextUpdateTime = now.AddMinutes(2);
+        }
+        else if (cacheIsValid == true && cacheExpiresAt.HasValue)
+        {
+            // Cache is valid - next update will be when cache expires OR next service check, whichever is later
+            // This accounts for the fact that the background service only checks every N minutes
+            nextUpdateTime = cacheExpiresAt.Value > nextServiceCheck ? cacheExpiresAt.Value : nextServiceCheck;
+        }
+        else if (cacheIsValid == false)
+        {
+            // Cache is invalid and not updating - next update will be at next background service check
+            nextUpdateTime = nextServiceCheck;
+        }
+
         var response = new RadarResponse
         {
             Frames = frames,
@@ -44,7 +75,7 @@ public static class ResponseBuilder
             CacheIsValid = cacheIsValid ?? false,
             CacheExpiresAt = cacheExpiresAt,
             IsUpdating = isUpdating,
-            NextUpdateTime = cacheExpiresAt ?? (isUpdating ? DateTime.UtcNow.AddMinutes(2) : null) // Estimate 2 min for update if in progress
+            NextUpdateTime = nextUpdateTime
         };
 
         return response;