fix: add stablecoin tracking to Polkadot treasury (#14231)

- Replace heroku-api export with direct implementation
- Add USDC and USDT tracking from Asset Hub and Hydration
- Properly categorize stablecoins in treasury breakdown
- Add @polkadot/api dependency to package.json
- Fixes issue where stablecoins showed as $0

Resolves #14231

Author: AntipasBen23