feat(settings): add API key management for merchant integrations

[frankrichard99]

Summary

Adds a complete UI for merchants to manage programmatic API keys at /settings/api-keys.

What's included

• List view: Displays all API keys with name, created date, last used date, and masked preview
• Create flow: Dialog to name and create a new key; displays the full key exactly once with a copy button
• Revoke flow: Confirmation dialog before permanently revoking a key
• Navigation: New "Settings" link in the sidebar with sub-navigation layout
• States: Loading, empty, error, and success states all handled

Files changed

File	Action	Description
lib/api.ts	Modified	Added ApiKey interface, listApiKeys, createApiKey, revokeApiKey
components/wallet/wallet-sidebar.tsx	Modified	Added Settings nav link
app/(app)/settings/layout.tsx	Created	Settings layout with sub-nav
app/(app)/settings/page.tsx	Created	Redirects to /settings/api-keys
app/(app)/settings/api-keys/page.tsx	Created	Full API key management page

Screenshots

(Add screenshots in the PR if possible)

Testing

• Verified TypeScript compilation with npm run build
• Tested create, copy, and revoke flows manually
• Confirmed responsive layout on mobile and desktop

Closes #498

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