fix(storage): the legacy profile migration is not atomic; a partial copy is taken for a finished one #1766

Problem

`AppDataRoot.migrateLegacySupportIfNeeded` skips the migration when `/querya_desktop/querya.db` exists, and `_copyDirectory` copies straight into the final directory in directory-listing order. If the copy stops halfway (disk full, the app is closed, a permission error), `querya.db` may already be there but incomplete. The next start sees it, skips the migration, and opens a truncated database. The order is arbitrary, so the database can also be copied without its `-wal` / `-shm` files.

Fix

Copy into a temporary sibling directory, copy `querya.db` last (with its `-wal`/`-shm` before it, or checkpoint first), then rename into place. Leave the legacy directory untouched. Test: a copy that fails midway leaves no `querya.db` in the new location and the next start migrates again.

Found in the system-functions review (see #1764).