fix(sync): recover when a suggested Nextcloud destination does not exist

[veryCrunchy]

Problem

Opening the native Nextcloud destination picker from a suggested media-sync path can start at a folder that does not exist yet. The WebDAV listing returns HTTP 404 and the picker leaves the person in an error state instead of guiding them through safe creation.

Expected behavior

• Represent folder-listing HTTP failures with typed status information rather than parsing error text.
• When the initial suggested destination is missing, recover to the nearest accessible parent.
• Clearly present the missing destination and let the person create it explicitly.
• Never create remote folders without confirmation.
• Preserve cancellation, offline-cache, path-validation, and traversal protections.
• Cover root, one-level, nested, permission failure, and cancellation behavior with tests.

Acceptance criteria

• A missing suggested path no longer produces a dead-end HTTP 404 card.
• The picker opens the nearest accessible parent and offers an explicit create-and-open action for the intended destination.
• Non-404 failures remain errors and are not treated as missing folders.
• Android and desktop implementations expose the same typed behavior.