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/Services/ApiClient.cs

@@ -173,6 +173,16 @@ public sealed class ApiClient : IDisposable
         return await response.Content.ReadFromJsonAsync<MessageDto>();
     }
 
+    public async Task<MessageDto?> SendUrlAsync(string channelName, string url)
+    {
+        EnsureAuthenticated();
+        var request = new SendUrlRequest(url);
+        var response = await AuthenticatedRequestAsync(() =>
+            _http.PostAsJsonAsync($"/api/channels/{Uri.EscapeDataString(channelName)}/send-url", request));
+        await EnsureSuccessAsync(response);
+        return await response.Content.ReadFromJsonAsync<MessageDto>();
+    }
+
     public async Task<ChannelDto?> CreateChannelAsync(string name, string? topic = null)
     {
         EnsureAuthenticated();