[Feature]: Implement Desktop external storage connections, sync, and reconcile

[Dencyuman]

Use case

Fulfill Desktop external storage, connection management, backend binding, sync/reconcile, and recovery workflows while preserving Runtime revision and Access guarantees.

Suggested shape

Implement the external StorageAdapter connection contract, OS keychain-backed credentials, provider capability discovery, remote picker/handoff, BackendBinding, conditional writes/leases, sync, reconcile preview, disconnect, and UI state.

Acceptance criteria

• Users can connect, authorize, inspect, refresh, and disconnect configured external storage adapters without credentials entering LDL, containers, logs, or frontend persistence.
• Remote selection resolves a stable Host Document ID and explicit BackendBinding distinct from the portable Project StableAddress.
• Open/save/autosave use provider capability negotiation, conditional writes or leases, and Runtime revisions rather than blind overwrite.
• External changes enter typed fast-forward, conflict, merge/rebase, or quarantined reconcile-preview paths and never silently replace restricted content.
• Runtime and Access re-evaluate definition, state, asset, package, and project-setting effects before publication.
• Offline edits, credential expiry, rate limit, partial upload, unknown write result, moved item, and reconnect converge to a recoverable state.
• Connection UI exposes provider, account, scope, last sync, pending work, conflict, and disconnect consequences without leaking secrets.
• Adapter contract tests and at least one deterministic reference adapter prove open/save/sync/reconcile/restart behavior.

Constraints

• Provider adapters must not reimplement LDL, Query, revision, conflict, or Access semantics.
• Do not add Organization/Workspace management, sharing, or realtime collaboration.

Alternatives and workarounds

Local files alone contradict the Desktop definition of opening local or connected LayerDraw documents and the Desktop F36/F37 matrix entries.

Dependency

Blocked by #128.