test: add Jest tests for walletService

[internet-dot]

Summary

Adds comprehensive Jest unit tests for walletService.ts.

Tests added

• Singleton: getInstance returns same instance
• Connection management: setConnection, getConnection, addListener, removeListener, isConnected
• disconnectWallet: clears connection and notifies listeners
• initialize: resolves without error
• getTokenBalances: native balance for ETH/MATIC/Arbitrum, USDC inclusion/skip/error handling, provider failure
• estimateGas: valid estimate, provider failure
• getWalletSigner: throws when no connection or no eip1193Provider
• createSuperfluidStream: user rejection (code 4001, ACTION_REJECTED), SFError formatting
• estimateSuperfluidCreateFlow: network mismatch
• createSablierStream: user denied via message text

All external dependencies (ethers, superfluid SDK, contracts, config) are mocked.

Fixes #14