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.
Research provenance: e4c113f.

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]

Someone is attempting to deploy a commit to the sivadaboi's projects Team on Vercel.

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[Omar-VF]

Thanks for this, and for #44–#46. One request: please move docs/research/CLOUD-LOCAL-46-LISTING.md out of docs/. That folder is the website (opensave.org), so anything in it gets published. The PR description or a comment on #46 is a good place for it instead. CI is approved and running.

[Seagulls]

Thanks — removed the internal research note from docs/ in a86d964. The PR description and #46 retain the scope, provenance, evidence, and the remaining mount-loss limitation. No Go source or test behavior changed. Focused go test, race-enabled tests, and go vet pass.