feat(sync): Sync pipeline — OneDrive download, upload, conflict handling

[jaybarden1]

Summary

Implement the full OneDrive sync pipeline as described in .claude/rules/onedrive-sync.md. This covers the six ordered pipeline steps, parallel job execution, conflict detection/resolution, and the background scheduler.

Depends on

• feat(wizard): Add Account wizard — OneDrive MSAL flow with Google/Dropbox stubs #23 — Add Account wizard (defines OneDriveAccount, IAuthService, IGraphService)
• Persistence issue — IAccountRepository, ISyncedItemRepository, ISyncRuleRepository, IDriveStateRepository, ISyncRepository

Scope

Pipeline steps (in order)

1. RemoteFolderEnumerator — fetch DeltaItem list from selected remote folders
2. RemoteDeletionDetector — delete local files whose remote counterpart is gone
3. LocalDeletionDetector — delete remote files the user deleted locally
4. DownloadJobBuilder — build download jobs (eTag-then-timestamp decision logic)
5. LocalChangeDetector — build upload jobs for new/changed local files
6. JobExecutor → ISyncPipeline → parallel workers via bounded Channel<SyncJob>

Key services

Interface	Notes
ISyncService	SyncAccountAsync, ResolveConflictAsync, events
ISyncScheduler	System.Threading.Timer, 60 min default, re-entrancy guard
ISyncPipeline	Bounded channel, workerCount from account.SyncConfig.WorkerCount
ISyncWorker / ISyncWorkerFactory	Per-worker execution
IHttpDownloader	Pre-signed URL download with retry/backoff
IUploadService	Resumable upload sessions, 10 MB chunks, retry/backoff
ISyncedItemRegistrar	Folder registration during enumeration
SyncRuleEvaluator	Most-specific-wins, default-deny
FileClassifier	Keyword tokenisation against FileClassificationRule
ConflictResolver / ConflictApplier	ConflictPolicy enum: KeepLocal / KeepRemote / Skip

Events

• ISyncService.SyncProgressChanged (SyncProgressEventArgs)
• ISyncService.JobCompleted (JobCompletedEventArgs)
• ISyncService.ConflictDetected (SyncConflict)
• ISyncScheduler.SyncStarted / SyncCompleted

ViewModels subscribe to these events and marshal to UI thread via RxApp.MainThreadScheduler.

Error types

• SyncError discriminated union: SyncAuthError, SyncGraphError, SyncStorageError, NoFoldersConfiguredError, SyncCancelledError

File system

All file I/O via IFileSystem (Testably abstraction). System.IO.* banned.

Acceptance criteria

• Sync runs for an OneDrive account and downloads new/changed files.
• Upload detects locally changed files and pushes them to OneDrive.
• Conflicts detected and surfaced via ConflictDetected event.
• Background scheduler fires every 60 minutes; manual trigger works.
• Re-entrancy guard prevents overlapping sync passes.
• Per-account cancellation works.
• Upload uses resumable sessions with 10 MB chunks and retry backoff.
• Download honours eTag-then-timestamp decision logic.
• dotnet build zero warnings; dotnet test all pass (unit + integration).

[jaybarden1]

partially completed by PR #53
missing features / incomplete features will be addressed in new issues - this one, in hindsight was too large