feat(contracts): Allow network editing with caution messaging on contract detail page

[NUMBER72857]

Summary

Users can now change a contract's network directly from the detail page. A contextual warning dialog explains the impact on duplicate detection and alert history before the change is committed.

Changes

• components/NetworkEditField.tsx:
  • Inline edit toggle — shows read-only view with Edit button, or select input
  • alertdialog warning with network-specific caution message before save
  • onNetworkChange(network) callback only fires after explicit confirmation
  • disabled prop for use while parent is saving
• __tests__/NetworkEditField.test.tsx: 8 tests covering all states and interactions

Integration in app/contracts/[id]/page.tsx

import { NetworkEditField } from '@/components/NetworkEditField';

<NetworkEditField
  currentNetwork={contract.network}
  onNetworkChange={async (net) => {
    await updateContract(contract.id, { network: net });
  }}
/>

Closes #87

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