feat(strategy): add quota-awareness to hybrid account selection

The hybrid strategy now considers account quota levels when selecting
accounts, preventing any single account from being drained to 0%.

- Add QuotaTracker class to track per-account quota levels
- Exclude accounts with critical quota (<5%) from selection
- Add quota component to scoring formula (weight: 3)
- Fall back to critical accounts when no alternatives exist
- Add 18 new tests for quota-aware selection

Scoring formula: Health×2 + Tokens×5 + Quota×3 + LRU×0.1

An attempt at resolving  badrisnarayanan/antigravity-claude-proxy#171

src/account-manager/strategies/trackers/index.js

@@ -6,3 +6,4 @@
 
 export { HealthTracker } from './health-tracker.js';
 export { TokenBucketTracker } from './token-bucket-tracker.js';
+export { QuotaTracker } from './quota-tracker.js';