feat: Implement end-to-end encryption for channels

- Added EncryptionSalt and WrappedRoomKey properties to Channel model.
- Introduced RoomCrypto class for client-side encryption and decryption.
- Updated ChannelService to handle encrypted channels, including creation and rekeying.
- Modified ChannelsController to expose crypto metadata and rekey functionality.
- Enhanced IrcCommandHandler to block joining encrypted channels over IRC.
- Updated database schema with migration for new encryption fields.
- Refactored file validation and image processing services to accommodate encrypted channels.
- Added unit tests for RoomCrypto functionality and updated existing tests for channel services.

src/EchoHub.Client/UI/Chat/ChatMessageManager.cs

@@ -255,6 +255,22 @@ public sealed class ChatMessageManager
                             lines.Add(new ChatLine($"       {trimmed}"));
                     }
                 }
+
+                // Clickable action to download the original image below the ASCII art
+                if (message.AttachmentUrl is not null)
+                {
+                    var imageName = message.AttachmentFileName ?? "image";
+                    var imageSize = FormatFileSize(message.AttachmentFileSize);
+                    var saveLine = new ChatLine(new List<ChatSegment>
+                    {
+                        new("       ", null),
+                        new($"[↓ save original] {imageName} [{imageSize}]", ChatColors.FileAttr),
+                    });
+                    saveLine.AttachmentUrl = message.AttachmentUrl;
+                    saveLine.AttachmentFileName = imageName;
+                    saveLine.Type = MessageType.Image;
+                    lines.Add(saveLine);
+                }
                 break;
 
             case MessageType.Audio:

src/EchoHub.Client/UI/MainWindow.cs

@@ -60,7 +60,7 @@ public sealed partial class MainWindow : Runnable
     private static readonly string[] SlashCommands =
     [
         "/status", "/nick", "/color", "/theme", "/send",
-        "/avatar", "/profile", "/servers", "/join", "/leave",
+        "/avatar", "/profile", "/servers", "/join", "/passwd", "/leave",
         "/topic", "/users", "/kick", "/ban", "/unban",
         "/mute", "/unmute", "/role", "/nuke", "/test-sound", "/quit", "/help"
     ];
@@ -154,6 +154,11 @@ public sealed partial class MainWindow : Runnable
     /// </summary>
     public event Action<string, string>? OnFileDownloadRequested;
 
+    /// <summary>
+    /// Fired when the user activates an image's "[save original]" line. Parameters: attachmentUrl, fileName.
+    /// </summary>
+    public event Action<string, string>? OnImageSaveRequested;
+
     /// <summary>
     /// Fired when the user activates a username (in userlist or message). Parameter is the username.
     /// </summary>
@@ -466,6 +471,13 @@ public sealed partial class MainWindow : Runnable
                 e.Handled = true;
                 return;
             }
+
+            if (line.Type == MessageType.Image)
+            {
+                OnImageSaveRequested?.Invoke(line.AttachmentUrl, line.AttachmentFileName);
+                e.Handled = true;
+                return;
+            }
         }
 
         var lineText = line.ToString();