3. Add `lib/utils/format.ts` number formatting

[DeFiVC]

What: Add number formatting utilities for displaying token amounts, percentages, and large numbers.

Why: Numbers need consistent formatting (1,234.56 LEARN, 75%, 1.2K users). Ad-hoc formatting is inconsistent.

Scope: Add functions for: formatTokenAmount, formatPercentage, formatLargeNumber, formatAddress. Use Intl.NumberFormat where possible.

Acceptance criteria:

• Token amounts are formatted
• Percentages are formatted
• Large numbers are abbreviated
• Addresses are truncated

Technical context: src/lib/utils/format.ts — format utils. src/components/wallet/balance-display.tsx — balance display.

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