feat: Add, update, remove holdings with validation

[michaelvic123]

This PR adds features to manage holdings in portfolios:

• Add holding to portfolio with symbol, quantity, cost basis, and chain
• Update holding (quantity, cost basis, rebalancing)
• Remove holding from portfolio
• Validate asset symbols (3-10 chars)
• Support multiple chains (Ethereum, Polygon, Binance, Solana)
• Update portfolio value calculations on holding changes
• Track cost basis for tax calculations
• Prevent duplicate holdings (same symbol + chain)
• Includes unit tests for all holding operations

Closes #31