feat: Implement message encryption and decryption support

- Added IMessageEncryptionService and its implementation MessageEncryptionService for handling message encryption.
- Updated ChannelsController and ChatService to encrypt messages before storing and sending.
- Introduced encryption key retrieval endpoint in ServerController.
- Modified EchoHubDbContext to accommodate increased message content and embed JSON lengths for encrypted data.
- Created migrations to support encryption-related database changes.
- Enhanced FirstRunSetup to ensure encryption key is generated if not present.
- Updated appsettings.example.json to include encryption configuration.
- Added comprehensive unit tests for encryption service and compatibility tests between client and server encryption.

src/EchoHub.Client/Services/ApiClient.cs

@@ -128,6 +128,16 @@ public sealed class ApiClient : IDisposable
         return info;
     }
 
+    public async Task<string> GetEncryptionKeyAsync()
+    {
+        EnsureAuthenticated();
+        var response = await AuthenticatedGetAsync("/api/server/encryption-key");
+        await EnsureSuccessAsync(response);
+        var result = await response.Content.ReadFromJsonAsync<EncryptionKeyResponse>()
+            ?? throw new InvalidOperationException("Server returned empty encryption key response.");
+        return result.Key;
+    }
+
     public async Task<UserProfileDto?> GetUserProfileAsync(string username)
     {
         EnsureAuthenticated();