SMBClient throws ErrorResponse with NTStatus.objectNameCollision (0xC0000035)
when a file already exists on the NAS. BackupError.uploadFailed wraps this,
hiding the real cause from the retry/failure logic.
Add isAlreadyExistsError() that unwraps BackupError.uploadFailed and checks
the underlying error for "object name collision", "already exist", or POSIX
EEXIST (code 17). When detected inside the retry catch block, the asset is
immediately marked as skipped and added to manifestEntries — no 3-retry
penalty (saves up to 6s per already-existing file), no false failure count.
The post-retry fileExists fallback is retained for edge cases where the file
lands on NAS but the error wasn't a collision (e.g. connection reset after
partial upload).
Also switch BackupQueueItem.UploadError to store the underlying transport
error's domain and description instead of the BackupError wrapper text, so
the SyncView failure list shows the real reason (e.g. "Access Denied",
"IO Timeout") rather than the generic "Failed to upload <file>".
Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>
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