perf(services): keep only the shared startup inventory read (J1)

Drop the startup deferral gate, its kill switch and the deferred loads:
they lowered renderer.ipcCallsToFirstCard but moved neither
spawnToFirstCardMs nor load->card beyond drift, and they grew
renderer.initialBytes. Keep the shared in-flight inventory read, inline
it in PlaylistsService with short property names, and copy the joiner's
result with structuredClone (the Electron renderer has it; the services
test setup polyfills it for jsdom).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/shared/interfaces/src/lib/electron-api.interface.ts

@@ -710,11 +710,6 @@ export interface ElectronBridgeApi {
     ) => () => void;
     getAppVersion: () => Promise<string>;
     platform: string;
-    /**
-     * `IPTVNATOR_DISABLE_STARTUP_DEFERRAL=1` kill switch: issue deferred
-     * startup work immediately instead of after the first rendered screen.
-     */
-    startupDeferralDisabled?: boolean;
     getAppUpdateStatus: () => Promise<ElectronBridgeAppUpdateStatus>;
     checkForAppUpdate: () => Promise<ElectronBridgeAppUpdateStatus>;
     downloadAppUpdate: () => Promise<ElectronBridgeAppUpdateStatus>;