feat: implement balance command for ERC20 token balances

[broody]

Summary

• Add controller balance [symbol] command that queries ERC20 token balances for the active session account
• Built-in support for 7 mainnet tokens: ETH, STRK, USDC, USD.e, LORDS, SURVIVOR, WBTC
• Custom tokens via controller config set token.<symbol> <address> (extends config with tokens map)
• All balance queries fire concurrently via tokio::spawn + futures::join_all
• 30s file-based cache to avoid redundant RPC calls
• Human-readable output (0.001 ETH) by default, structured JSON with --json
• Zero balances hidden when querying all tokens

Usage

# Query all non-zero balances
controller balance --chain-id SN_MAIN

# Query specific token
controller balance eth --chain-id SN_MAIN

# Add custom token
controller config set token.custom 0x1234...

# JSON output
controller balance --json --chain-id SN_MAIN

Test plan

• controller balance --chain-id SN_MAIN returns non-zero balances
• controller balance eth --chain-id SN_MAIN returns single token
• controller balance --json --chain-id SN_MAIN returns structured JSON
• Repeated calls within 30s hit cache (instant response)
• controller config set token.custom <addr> then controller balance custom works
• Unknown token symbol returns helpful error

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