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.Core/DTOs/ServerDtos.cs

@@ -1,15 +1,3 @@
 namespace EchoHub.Core.DTOs;
 
-public record ServerInfoDto(
-    Guid Id,
-    string Name,
-    string? Description,
-    string Url,
-    int OnlineUsers,
-    int TotalChannels,
-    bool IsOnline,
-    DateTimeOffset LastPingAt);
-
-public record RegisterServerRequest(string Name, string Url, string? Description = null);
-
 public record ServerStatusDto(string Name, string? Description, int OnlineUsers, int TotalChannels);