Use balances interval

[Adeyemi-cmd]

Close #142

PR Description

Summary

Replaces the manual setInterval(fetchBalances, 30000) in useBalances with @tanstack/react-query's useQuery to properly pause polling when the tab is backgrounded.

Problem

setInterval(fetchBalances, 30000) runs regardless of page visibility. The callback fires even when the tab is in the background, wasting network requests and battery.

Solution

• Use useQuery with refetchInterval: 30000 and refetchIntervalInBackground: false (default) — polling auto-pauses when the tab is hidden and resumes when visible.
• Use enabled: !!address && connected to skip all requests when the wallet is disconnected.
• Extract fetchBalancesFromHorizon as a standalone async function.
• Update tests to mock @tanstack/react-query (consistent with other hooks in the codebase).

Acceptance Criteria

• Polling pauses when tab is backgrounded
• Polling resumes when tab becomes visible
• No unnecessary network requests on disconnected state
• All existing tests pass

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[K1NGD4VID]

Resolve Merge confict @Adeyemi-cmd

[K1NGD4VID]

Resolve Merge Conflict @Adeyemi-cmd