feat: add offline experience with tiered cache and network monitoring

[BrunoCerberus]

Summary

• Add persistent disk cache (L2) alongside existing in-memory cache (L1) with 24-hour TTL for offline content availability
• Add NetworkMonitorService using NWPathMonitor to track connectivity and display an animated offline banner
• Implement graceful degradation: cached content stays visible on failed refresh, offline-specific error views with wifi.slash icon, and typed PulseError.offlineNoCache error
• Update documentation (README, CLAUDE.md, AGENTS.md) to reflect the new caching and offline architecture

Details

Tiered Cache

Layer	Implementation	TTL	Survives App Kill
L1 (Memory)	LiveNewsCacheStore (NSCache)	10 min	No
L2 (Disk)	DiskNewsCacheStore (JSON in Caches/)	24 hours	Yes

Fetch Flow

1. L1 hit (non-expired) → return
2. L2 hit (non-expired) → promote to L1, return
3. Offline → serve stale from L1/L2; if nothing → PulseError.offlineNoCache
4. Online → network fetch → write-through L1+L2; on failure → stale L2 fallback

New Files

• PulseError.swift - Typed offline error enum
• NetworkMonitorService.swift - Protocol + Live (NWPathMonitor) + Mock
• DiskNewsCacheStore.swift - Persistent file-based cache
• OfflineBannerView.swift - Animated offline banner
• DiskNewsCacheStoreTests.swift - 8 unit tests for disk cache

Modified (key files)

• CachingNewsService.swift - Tiered L1+L2 cache with fetchWithTieredCache() + offline awareness
• HomeDomainInteractor+DataLoading.swift / MediaDomainInteractor / SearchDomainInteractor - Offline error detection, preserve data on failed refresh
• CoordinatorView.swift - Offline banner integration
• HomeView / MediaView / SearchView - Show cached data over error, offline-specific error views

Test plan

• DiskNewsCacheStoreTests - write/read, TTL, remove, corrupted file, overwrite (8 tests)
• Existing CachingNewsService*Tests updated for new init signature
• HomeViewStateReducerTests and HomeViewModelTests updated for isOfflineError field
• Manual: Kill app → airplane mode → relaunch → cached content visible from disk
• Manual: Airplane mode while viewing → banner appears, pull-to-refresh keeps content
• Manual: First launch on airplane (no cache) → offline error with wifi.slash icon
• Manual: Restore connectivity → banner disappears, refresh fetches fresh data

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