fix(16-01): update network error message per REQ-5.3

- Change uploadAttachment error message to 'Upload failed. Please try again.'
- Provides actionable feedback to retry on network errors
- Matches exact wording from REQ-5.3

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

@@ -90,7 +90,7 @@ export const uploadAttachment = createAsyncThunk(
       if (error.status === 422) {
         return rejectWithValue('Invalid file type or format');
       }
-      return rejectWithValue(error.message || 'Upload failed');
+      return rejectWithValue(error.message || 'Upload failed. Please try again.');
     }
   }
 );