test(wallet): cover useWalletWatcher account/network change handling

[codemagician1949]

#358 asks for Freighter account/network change listeners that update the auth store and notify the user. That is already implemented (watchWalletChanges in lib/stellar/wallet.ts plus the useWalletWatcher hook), but nothing tested it. Adds use-wallet-watcher.test.tsx covering: no watcher while disconnected, account change (toast + disconnect), network change (store update + toast), an unchanged report being ignored, and the watcher being stopped on unmount. npx vitest run src/lib/hooks/use-wallet-watcher.test.tsx: 5/5 passing. No production code changed.

To be completed (not part of this PR, not closed by it)

• 0. Add lib/stellar/contracts.ts contract interaction helpers #359 — lib/stellar/contracts.ts helpers (file exists; not reviewed against the acceptance criteria)
• 8. Add app/credentials/page.tsx credentials list #357 — app/credentials/page.tsx (page exists; not reviewed)
• 7. Add app/rewards/page.tsx rewards dashboard #356 — app/rewards/page.tsx (page exists; not reviewed)

Closes #358

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