Write unit tests for formatting utilities

[Code-Paragon]

Problem

The src/utils/formatting.ts file handles critical BigNumber conversions (e.g., formatStroops), but has no test coverage to prove it protects against precision loss or invalid inputs.

Expected behavior

A test suite is added that explicitly passes edge-case inputs (large numbers, zero, NaN strings) into formatStroops, formatCurrency, and truncateAddress to verify robust behavior.

Files to update

• src/utils/formatting.test.ts (create)

Project relevance

Mathematically guarantees that the UI will not crash or display incorrect balances when receiving unpredictable XDR values from the blockchain.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• Test asserts that formatStroops("invalid") safely returns "0.00"
• No TypeScript, lint, test, or build errors