Offline constant lookups from cached metadata

Problem

dot const (src/commands/const.ts:29) creates a chain client (live connection), but constants are embedded in metadata — they don't need a live chain. This means dot const fails when the chain is unreachable, even though the data is already cached locally.

Proposal

When cached metadata exists for the target chain, decode the constant directly from metadata without establishing a connection.

# Should work offline after first use
dot const Balances.ExistentialDeposit --chain polkadot

Implementation

• Check if cached metadata exists for the target chain before connecting
• If cached metadata is available, decode the constant value directly from it
• Fall back to live connection only if no cache exists
• Consider adding a --offline flag to explicitly require offline mode (error if no cache)

Acceptance Criteria

• dot const works without a live chain when metadata is cached
• Falls back to live connection when no cache exists
• Output is identical whether online or offline

Start in src/commands/const.ts:29 and trace how cached metadata and the live chain client are obtained. Run dot const Balances.ExistentialDeposit --chain polkadot with and without connectivity, then verify cached metadata is used offline, live lookup remains the fallback, and both paths produce identical output.