Replace the sequential for-loop with withTaskGroup running N concurrent
uploads. Each slot gets its own SMB/SFTP connection so uploads never
block each other. Default concurrency is 3 (configurable via
ConnectionStore.maxConcurrentUploads, capped at 6).
- ConnectionStore: add maxConcurrentUploads (default 3, persisted)
- BackupEngine: build pool of N connections at start, seed TaskGroup
with one task per slot, recycle slots as tasks complete
- Manifest checkpoints, pause, cancel, retry logic all preserved
- SlotResult extracted to BackupEngine extension for Sendable conformance
- Fix: remove undefined .trackScrollForTabBar() from BackupView
Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>
8fb425344d