Stellar: Implement XLM price fluctuation rate buffer locks

[sublime247]

Difficulty: Medium
Skills: TypeScript, Rate controllers
Files: src/services/currency.ts, src/controllers/rateController.ts

Description:
Apply a dynamic price buffer limit to protect the platform from ledger fee fluctuations during market volatility.

Acceptance Criteria:

• Lock exchange rate for 60 seconds on calculator load.
• Apply 0.5% buffer threshold to absorb mid-transaction fluctuations.
• Recalculate rate automatically if the lock expires.

Questions? Comment below!