Add cross-module integration tests

[marcstraube]

Context

All modules have thorough unit tests (99%+ coverage), but no integration tests verify cross-module interactions.

Goal

Integration tests for key workflows to increase confidence in module interactions.

Implementation

Test these combinations:

• EncryptedStorage + StorageManager
• RequestInterceptor + RetryQueue
• OfflineQueue + IndexedDB + NetworkStatus
• Cache + Storage (stale-while-revalidate flow)

Files

• tests/integration/ (new directory)

Source: AUDIT2 (-1 Test Quality), AUDIT3 (implicit)