fix: preserve in-progress progress during startup state sync

[aby913]

Summary

• preserve existing progress only for downloading -> downloading updates when incoming snapshot/state-sync data carries empty progress
• align CacheManager change detection with write-time merge logic to avoid startup regressions
• apply the same downloading-only normalization in StateMonitor progress comparison to prevent false app_state_change pushes

Why this scope

The previous broader in-progress-state fallback could retain stale progress in non-downloading states. This update narrows behavior to the only accepted case:

• keep progress only when both old/new states are downloading and incoming progress is empty
• do not preserve progress for downloadingCanceling, downloadingCanceled, or other states

Validation

• go test ./internal/v2/appinfo ./internal/v2/utils