[Lee-Juntong]

On Windows the decrypted mirror contains database files, while attachments remain in the original WeChat account. media/openfile currently derive their attachment root from the mirror, so existing documents and media cannot be found there.

Resolve the original account directory by matching the selected mirror account against the original data roots. Fail explicitly when that original directory cannot be uniquely located, then use it for both media export and document lookup. Keep the existing direct-store behavior on the SQLCipher backend.

Validation: five synthetic tests cover matching the correct account among multiple accounts, missing original data, direct-store behavior, media routing, and reading a document present only in the original directory. All five fail on main and pass with this change. Filename-collision handling remains in the separate existing PR #5.

Local validation uses Windows/Python 3.12 and synthetic fixtures. The consistency-checker file and two fingerprint-dependent existing tests were excluded by the local execution policy; no account databases, chat contents or keys are included.

Local branch suite: 86 passed, two tests deselected, consistency-checker file excluded.

[freanocank]

Thanks for the fix and regression tests. Adopted through #15 with your original commits preserved. Attachment lookup now uses the matching original Windows account, and the synthetic routing/document regressions pass.

The combined suite passes all four macOS/Windows × Python 3.10/3.13 CI jobs; Windows reports 209 passed, and macOS reports 208 passed with the Windows-only test skipped.