feat(14-01): extend WebSocket message transformation for attachment fields

- Add attachment fields to CHAT_MESSAGE handler transformation
- Include attachmentId, attachmentName, attachmentType, purpose, attachmentSize
- Fields null/undefined for regular text messages
- WebSocket payload maps to Redux format with attachment metadata

jam-ui/src/components/client/JKSessionScreen.js

@@ -583,7 +583,13 @@ const JKSessionScreen = () => {
             message: payload.msg,
             createdAt: payload.created_at,
             channel: payload.channel || 'session',
-            sessionId: currentSession.id
+            sessionId: currentSession.id,
+            // Attachment fields (null/undefined if not an attachment)
+            purpose: payload.purpose,              // 'Notation File', 'Audio File', or undefined
+            attachmentId: payload.attachment_id,   // MusicNotation UUID
+            attachmentType: payload.attachment_type, // 'notation' or 'audio'
+            attachmentName: payload.attachment_name, // filename
+            attachmentSize: payload.attachment_size  // file size in bytes (may be null)
           };
           console.log('[CHAT DEBUG] Dispatching to Redux:', message);
           dispatch(addMessageFromWebSocket(message));