fix: address save-to-device review findings

[zerox80]

Summary

• keep export downloads isolated from the shared local cache and Room state
• retry temporary network and server failures without deleting the persisted export job
• persist descendant checkpoints so large selected folders resume where they stopped
• update the export-job Room schema and the affected view-model test setup

Why

Follow-up to the review of opencloud-eu#206. This prevents cache races, preserves retry state, and avoids restarting a large folder from its first child.

Checks

• static diff and schema inspection
• tests and build not run, per repository instruction

[zerox80]

@junkerderprovinz pls merge this then its Merge-Ready.

[junkerderprovinz]

Thanks for this - the export-isolation + retry-checkpoint work is exactly what was needed after the opencloud-eu#206 review. Appreciate the follow-through on both rounds.