fix: prevent stale state updates in useStellarBalance after unmount

[mertcicekci0]

Summary

• useStellarBalance hook fires async RPC calls (balance + decimals + metadata) in a useEffect but has no cleanup — if the component unmounts or address changes mid-flight, the
  resolved promises still call setState on a stale component
• Added a cancelledRef that is set to true on effect cleanup, checked before every state update in both refreshBalance and fetchAssetMetadata
• Standard React pattern for preventing state updates after unmount

Changes

File	Change
packages/paywall/src/browser/useStellarBalance.ts	Add useRef cancellation flag + effect cleanup

Test plan

• pnpm --filter @x402-stellar/paywall typecheck passes
• Connect/disconnect wallet rapidly — no React "setState on unmounted component" warnings
• Balance still loads correctly on normal wallet connection flow

[Copilot]

Pull request overview

This PR adds a cancellation mechanism to the useStellarBalance React hook to prevent stale state updates when the component unmounts or the address prop changes while async RPC calls are in-flight. This follows the standard React pattern of using a ref-based cancellation flag tied to effect cleanup.

Changes:

• Introduces a cancelledRef (via useRef) that is set to false when the effect runs and true on cleanup
• Guards setState calls in both refreshBalance and fetchAssetMetadata callbacks with cancelledRef.current checks
• Adds proper effect cleanup return function in the useEffect that triggers balance fetching

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[Copilot]

The cancellation guard at line 142 returns early from the try block, and the one at line 150 returns early from the catch block, but the finally block (line 160-162) still calls setIsFetchingBalance(false) unconditionally. Since finally always runs regardless of early returns, this state update will still fire after unmount/cancellation, undermining the purpose of these guards. You should add a cancelledRef.current check inside the finally block as well, or restructure so that the setIsFetchingBalance(false) call is placed before each return instead of in finally.