Implement daily limit enforcement for testnet faucet requests

[Parsh]

This pull request introduces a per-app daily request limit to the testnet faucet flow in the Keeper app and relay. The main goal is to prevent a single app instance from draining the faucet by capping requests to 5 per appId per UTC day. The implementation includes backend quota enforcement, structured error handling, and improved client UX for quota-reached scenarios. Both backend and frontend are updated to support this new quota, with clear user messaging and robust error discrimination.

Backend (Relay) changes:

• Daily quota enforcement: The /testnetFaucet endpoint now requires appId and enforces a maximum of 5 successful requests per appId per UTC day. Exceeding the limit returns HTTP 429 with a structured error code FAUCET_DAILY_LIMIT_REACHED. The quota counter is incremented only after a successful transfer.

Frontend (Keeper app) changes:

• Client quota awareness and error handling: The app now includes appId in faucet requests, distinguishes quota errors (HTTP 429) from generic failures, and sets a dedicated Redux flag (testCoinsQuotaReached) when the daily limit is hit.
• User experience improvements: When the daily limit is reached, a KeeperModal is shown with clear messaging and an OK button, keeping the user on the settings screen. The Receive Test Sats row now displays a static hint about the daily limit.

Specification and documentation:

• OpenSpec documentation: Detailed context, design decisions, requirements, and tasks are documented to clarify the quota logic, error handling, and affected files.

Most important changes:

Backend: Quota Enforcement

• /testnetFaucet now enforces a per-appId daily limit (max 5 requests per UTC day), returning HTTP 429 with a structured error code if exceeded, and increments the quota only after successful transfers.

Frontend: Client Handling and UX

• Keeper app includes appId in requests, distinguishes quota errors, and manages a new Redux flag for quota-reached state.

Localization

• Adds English and Spanish string keys for all new quota-related UI messages.

Specification/Documentation

• Thorough OpenSpec documentation details context, requirements, design decisions, and implementation tasks for the new quota system.