[GhosTnever-lkm]

What changed

• Align JSON backup export and import on a 20 MiB limit measured in UTF-8 bytes.
• Use the same shared limit in the browser file-upload guard and core parser/exporter.
• Add a regression test covering 5,000 allowed shifts with 500-character control-character notes, which previously produced a 3.4 MB export that could not be imported.
• Update the README and changelog for v0.1.3.

Verification

• npm.cmd run check — 10/10 tests passed.
• git diff --check — passed.
• Reproduced the old failure before the change: 5,000 valid shifts exported to 3,385,060 bytes and parseBackup rejected the file as BACKUP_TOO_LARGE.