Task : Interactive Wallet Connection & Mock Balance State
The Assignment: Replace the static "Connect Wallet" button in Header.tsx with a functioning simulated Web3 connection flow:
 - Clicking "Connect Wallet" opens a modal showing 3 options: MetaMask, Phantom, Coinbase Wallet.
 - Connecting updates global app state to show a truncated address (e.g., 0x7a...4B2e), an avatar, and an ETH balance (e.g., 2.45 ETH).
 - Persist connection state across page refreshes (via localStorage or cookie).
 - Clicking the connected address opens a dropdown with "Copy Address", "Disconnect", and an active network badge ("Ethereum Mainnet").