fix(16): fix attachment deduplication in uploadAttachment.fulfilled

The previous fix only added deduplication by attachmentId in addMessageFromWebSocket
and fetchChatHistory.fulfilled, but missed the uploadAttachment.fulfilled handler.

Race condition: WebSocket message can arrive BEFORE the upload API returns.
When this happens:
1. WebSocket delivers message with id='456', attachmentId=123
2. addMessageFromWebSocket adds it to state
3. Upload API returns
4. uploadAttachment.fulfilled only checked for id='attachment-123' → not found
5. Duplicate message added

Fix: Also check by attachmentId in uploadAttachment.fulfilled handler.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

jam-ui/src/store/features/sessionChatSlice.js

@@ -468,9 +468,13 @@ const sessionChatSlice = createSlice({
           // Use music_notation.id as part of message id to avoid collision with real msg_id
           const messageId = `attachment-${notation.id}`;
 
-          // Check for duplicates (in case WebSocket somehow delivers to sender)
-          const exists = state.messagesByChannel[channel].some(m => m.id === messageId);
-          if (!exists) {
+          // Check for duplicates by both message ID AND attachmentId
+          // WebSocket may deliver message before this handler runs with different ID format
+          const existsById = state.messagesByChannel[channel].some(m => m.id === messageId);
+          const existsByAttachmentId = state.messagesByChannel[channel].some(
+            m => m.attachmentId && m.attachmentId === notation.id
+          );
+          if (!existsById && !existsByAttachmentId) {
             const purpose = attachmentType === 'audio' ? 'Audio File' : 'Notation File';
             const message = {
               id: messageId,