test: cover mobile service layer and sync queue

[brentthomas248]

Summary

• Add mocked-fetch coverage for the mobile aid API service, including list/detail reads, claim submission headers, and error statuses.
• Add AsyncStorage-backed aid cache tests for empty cache, persistence, timestamp writes, and invalidation.
• Add offline sync queue network tests for offline queueing, online flush, retryable failures, persisted errors, and saver-mode flush limits.

Closes #104

Verification

• PASS: npm test -- --runInBand src/__tests__/api.test.ts src/__tests__/claimSubmissionQueue.test.tsx src/__tests__/aidApi.test.ts src/__tests__/aidCache.test.ts src/__tests__/syncQueueNetwork.test.ts (5 suites, 33 tests)
• PASS: npx eslint src/__tests__/aidApi.test.ts src/__tests__/aidCache.test.ts src/__tests__/syncQueueNetwork.test.ts (only existing Node module-type warning from eslint.config.js)
• PASS: git diff --check

Existing Baseline Failures Not Introduced Here

• npm test -- --runInBand currently fails existing suites: navigation.test.tsx, screens.test.tsx, HomeScreen.test.tsx, and ScannerScreen.test.tsx (ExpoBarCodeScanner native module missing in Jest setup, theme provider/test harness errors). The new service/cache/sync tests pass in the full run.
• npm run lint currently fails on an existing ReleaseNotesModal.tsx react/no-unescaped-entities error plus existing warnings outside this patch.
• npx tsc --noEmit --project tsconfig.json currently fails on existing AsyncStorage.clear typing in claimSubmissionQueue.test.tsx and existing theme/accessibility type errors in AidDetailsScreen.tsx / ClaimReceiptScreen.tsx.