[Studio] fix: connect K8s certificate page to APIs

[btlqql]

Summary

• load K8s certificates through the service layer instead of importing page-level mock data
• connect create, update, and delete actions to the existing backend endpoints
• add loading, submitting, validation, SAN parsing, and request error states
• keep mock-mode certificate CRUD consistent through an in-memory store
• return created, updated, and renewed records from the API wrappers
• add API contract coverage for list, create, update, and delete operations

Root cause

The certificate page initialized itself directly from mockK8sCerts and mutated only React state. Although the API and server controller already implemented certificate CRUD, production UI actions never invoked them, and the add button remained a placeholder.

Impact

Production users now see server-provided certificate data and certificate changes persist through the backend. Mock mode remains useful for local development, including create, edit, and delete flows.

Validation

• cd web && npm test — 10 tests passed
• cd web && npm run build — passed
• cd web && npm run lint — 0 errors; 3 pre-existing warnings in unrelated files
• pre-commit ESLint and Prettier checks — passed

Fixes #439