finish a chunked transfer when the length is an exact multiple

[MaciejZet]

Summary

A chunked save sets finished when the next offset is past the end of the payload. On an exact multiple of the chunk size that comparison stays false for the last block, so the background page never writes the file. The download fallback, the editor tab data, and the editor open path all used that comparison.

16 MiB still goes out as a single block. 32 MiB and 48 MiB now finish on the last block.

The fetch sender walks with <= and already marks the following empty block finished. That path is left as it is.

Test plan

• test/chunk-finished.test.js: exact 2× and 3× chunks, one unit under and over, a single chunk, an empty payload, 16 / 32 / 48 MiB, and 32 MiB ± 1
• npm test: 29 passed
• eslint on the touched files
• ./build.sh; the committed bundles include the same >= check

[gildas-lormeau]

Thanks! I removed chunkFinished and its test and kept the >= fix inline. It's a one-character change, so a helper module felt like too much.