fix(cloud/local): propagate destination listing errors

[Seagulls]

Refs #46 — partial fix only.

An unavailable or unreadable local-folder destination previously returned
an empty successful listing. This patch propagates the underlying os.ReadDir
error, allowing callers to distinguish a storage failure from an empty backup.

Tests cover absent paths, non-directory destinations and ordinary folders.

The mount-loss fallback-write problem is NOT fixed. Uploads can still create
a missing local directory, and detecting a vanished NAS mount requires a
separate opt-in mount-identity contract.

Based on dev-public at 7b69042.
Focused cloud tests, race detector and go vet pass.
The backend E2E suite passed during combined-branch validation.
The desktop app was not built because frontend/dist is absent.

[vercel]

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