fix(useWallet): handle Freighter network change events

[tapheret2]

Problem

useWallet refreshes on mount but may not subscribe to network/account change events, so the navbar can show a stale network after the user switches in the extension.

Proposal

Subscribe to Freighter (or wallet) change listeners when available; clean up on unmount.

Acceptance

• Switching network in extension updates network state without full reload
• No listener leak (remove on unmount)