Graceful Degradation with Feature Flags and Capacity Shedding

[Husten150]

Summary

Implements system-wide graceful degradation with feature flags and capacity shedding as described in issue #107.

Changes

Feature Flag System (src/lib/feature-flags.ts, src/components/FeatureFlagProvider.tsx)

• Defines all feature flags as a typed union (staking, governance, quadratic-voting, collateral, analytics, notification, explorer)
• URL parameter overrides: ?feature_staking=false
• localStorage persistence for dev toggling
• React context provider with useFeatureFlag() and useFeatureFlags() hooks
• Priority-based fallback: computeFeatureFlags() merges defaults, code overrides, localStorage, and URL params

Capacity Shedding (src/lib/capacity-shedding.ts, src/components/CapacitySheddingProvider.tsx)

• Monitors response time, queue size, error rate, and memory usage against configurable thresholds
• Three capacity levels: healthy -> degraded -> critical
• Priority-based shedding: critical features always available, low-priority shed first
• Periodic metrics collection with auto-adjusting capacity state
• subscribeToLevelChanges() for external monitoring integration

Graceful Degradation (src/components/DegradableFeature.tsx)

• Wraps any feature with automatic fallback UI when disabled or shed
• Shows "currently unavailable" message with feature name when degraded
• Customizable fallback via fallback prop

UI Indicators (src/components/CapacityIndicator.tsx)

• Shows banner when system is degraded or critical
• Color-coded: green (healthy, hidden), yellow (degraded), red (critical, animated)
• Expandable details explaining current system state

Client Broadcast Recovery Queue

• src/hooks/useTxRetryQueue.ts: Persistent retry queue with LRU eviction, duplicate detection, exponential backoff
• src/components/RetryWatcher.tsx: Background worker that retries pending transactions with polling
• src/components/PendingTransactionsBanner.tsx: UI banner showing pending count with expandable details
• src/components/Toast.tsx: Toast notification system for user feedback

Staking Integration

• src/hooks/useSorobanStaking.ts: Staking hook with retry queue integration and refresh recovery
• src/lib/stellar/rpcClient.ts: RPC client with proper PENDING/ERROR status handling
• src/lib/sessionStore.ts: SessionStorage wrapper for persistent queue state

Tests

• tests/graceful-degradation.spec.ts: Playwright tests for feature flags and capacity shedding
• tests/retry-queue.spec.ts: Playwright tests for retry queue and duplicate detection

Bug Fixes

• Stale closure in FeatureFlagProvider: Fixed onPopState callback to use useRef instead of capturing stale overrides from closure
• updateEntry with null txHash: Fixed recoverFromRefresh to properly update entries with null txHash
• sendTransaction always returning confirmed: Fixed rpcClient to check actual RPC response status
• Missing pending status handling: Added pending status handling in RetryWatcher and useSorobanStaking

Closes #107