test(frontend): add Svelte component tests for OAuth UI

[fpindej]

Problem

OAuth UI components (OAuthProviderButtons, ConnectedAccountsCard, DisconnectDialog, OAuthProviderButton) have no tests. The current vitest setup runs in node environment without DOM/jsdom, so Svelte component rendering tests are not possible.

Proposed approach

1. Add @testing-library/svelte and jsdom (or happy-dom) to dev dependencies
2. Configure a separate vitest environment for component tests (e.g., *.component.test.ts files use jsdom)
3. Write tests for:
   • OAuthProviderButtons: mock fetch, verify buttons render for returned providers, verify nothing renders when no providers
   • ConnectedAccountsCard: verify connect/disconnect button states based on linkedProviders and hasPassword props
   • DisconnectDialog: verify confirmation flow, unlink API call, error handling
   • OAuthProviderButton: verify icon rendering, loading/disabled states

Context

• Server-side logic is already tested (callback page.server.test.ts in PR test(auth): add OAuth external authentication tests #369)
• This requires DOM testing infrastructure that doesn't exist yet
• Consider setting up the DOM test infrastructure as a shared effort that benefits all future component tests, not just OAuth

Related

• Part of OAuth polish tracked in chore(auth): OAuth external login - polish and followups #364
• May benefit from Testcontainers infrastructure (test(backend): add Testcontainers integration test project #174) for integration-level tests later