fix(BalanceList): prevent duplicate React keys and add tests for loading skeleton and empty state (#601)

[stayzappy]

Closes #601

Summary

• Updated BalanceList.tsx sorted.map() key from key={b.asset} to composite unique keys to prevent React duplicate key warnings.
• Added unit tests in BalanceList.test.tsx:
  • Disconnected state connect prompt.
  • Loading state rendering exactly 3 skeleton rows.
  • Empty state rendering.
  • Loaded state rendering balance rows with formatted amounts.
  • Regression test verifying zero React duplicate key warnings during render.