Refactor EchoHub solution structure and remove unused components

- Deleted EchoHub.slnx and related project files for EchoHub.Web and EchoHub.Api.
- Removed ServerDirectoryDbContext, ServerDirectoryService, and related extensions and DTOs.
- Updated CommandHandler and Program.cs in EchoHub.Client to handle URL sending instead of file downloading.
- Added SendUrlRequest DTO and corresponding API endpoint in ChannelsController for handling URL uploads.
- Implemented database migration support in EchoHub.Server with initial migration setup.
- Adjusted Program.cs to include HTTP client configuration for image downloading.
- Cleaned up unnecessary files and references to streamline the project.

src/EchoHub.Client/Commands/CommandHandler.cs

@@ -129,8 +129,8 @@ public class CommandHandler
                 await OnSendFile(target);
             var fileName = Path.GetFileName(uri.LocalPath);
             if (string.IsNullOrWhiteSpace(fileName))
-                fileName = "download";
-            return new CommandResult(true, $"Downloading & uploading: {fileName}...");
+                fileName = "image";
+            return new CommandResult(true, $"Sending: {fileName}...");
         }
 
         if (!File.Exists(target))