Add Pagination Prefetch Tuning to Avoid Redundant /vaults Calls

[maugauwi-hash]

Area: Performance
Priority: Low
Estimated Time: 1.5 hours

Description:
refreshAll()/its iOS equivalent (from #112's pagination work) fetches all pages via
cursor-based pagination; confirm this isn't called more often than necessary (e.g., on every
pull-to-refresh and every WebSocket vault_updated event), which would generate redundant network
traffic for the common case where only one vault actually changed.

Tasks:

• Audit call sites of the full-refresh function vs. targeted single-vault refresh
• Prefer updating just the affected vault in local state when a vault_updated event names a specific
  vault ID, reserving full-list refresh for pull-to-refresh and initial load
• Add tests asserting a single-vault WebSocket update doesn't trigger a full /vaults re-fetch