test: cover error pages and shared recovery components

[jakerated-r]

Closes #594.

Summary

• add coverage for the app-level server error page, including reset, digest, and support/home recovery actions
• add coverage for the not-found page search field, home link, and router back recovery
• add coverage for the network-error page troubleshooting copy, retry disabled state, and failed connectivity recovery
• add shared ErrorLayout/ErrorButton coverage for internal links, external safe links, click handlers, and disabled buttons

Validation

• npm ci ✅
• npm test -- --run tests/error-page.test.tsx tests/not-found-page.test.tsx tests/network-error-page.test.tsx tests/transaction-error-page.test.tsx src/components/__tests__/ErrorLayout.test.tsx ✅ 5 files / 16 tests passed
• git diff --cached --check ✅

Repo-wide test note

• npm test -- --run is still red on existing unrelated repository issues, including duplicate symbols in src/lib/backend/validation.ts, src/lib/backend/services/contracts.ts, and src/lib/backend/auditLog.ts; malformed JSX in src/components/modals/CommitmentDetailsModal.tsx; missing @testing-library/user-event in an existing test; and non-DOM component tests without a DOM environment.

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