Unify VSCode extension on the shared @flapi/shared API client

[jrosskopf]

The extension ships its own src/shared/apiClient.ts (native fetch) in addition to @flapi/shared's FlapiApiClient (axios) and raw createApiClient. CLAUDE.md mandates a single shared client. Collapse onto @flapi/shared. The extension's local client also reads non-existent flapi.host/flapi.port settings (config is flapi.serverUrl), so it always falls back to localhost:8080 — fix as part of this. (Audit P2)