…out-of-bounds error

When loading remote DICOM files via manifest.json, chunks arrive incrementally
due to parallel asynchronous processing in importDataSources. This creates a
race condition where:

1. Initial chunks (e.g., 1-50) arrive and image is allocated for 50 slices
2. These chunks start loading their data
3. Additional chunks (51-100) arrive with some already having loaded data
4. Without fix: processNewChunks() attempts to write data at positions 51-100
   into an array only allocated for 50 slices, causing "offset is out of bounds"
5. With fix: reallocateImage() runs first, expanding the array to accommodate
   all chunks before any data is written

The fix ensures proper ordering:
- Move reallocateImage() call before processNewChunks()
- Fix index lookup in processNewChunks() to use actual chunk position
  instead of filtered array index

This prevents RangeError when chunks arrive in multiple batches with
pre-loaded data from parallel fetching.