Navbar wallet button and network switcher

[iexwr]

Context

The Navbar has no wallet connection or network control; users cannot switch Testnet/Mainnet or see their account.

Acceptance criteria

• Add a connect/disconnect button in src/components/Navbar.tsx wired to useWallet.
• Show a shortened pubkey (GABC...XYZ) when connected.
• Add a Testnet/Mainnet toggle that writes the network choice to localStorage and is read by the RPC/client config.

Technical notes

• Keep the toggle accessible (aria-label, keyboard operable).

Testing

• Component test: click connect (mocked), assert pubkey renders; toggle persists.