feat(engine): make AddNode non-blocking with async node creation #1039

Job	Run time
Plugins / Lint (NLLB)	2m 24s
Plugins / Lint (Whisper)	38s
E2E / Playwright E2E	16m 29s
Plugins / Lint (Sherpa-ONNX Plugins)	1m 6s
UI / Test	26s
Skit / Lint (fmt + clippy)	3m 49s
Skit / Build	11m 1s
UI / Lint (TypeScript + ESLint)	43s
UI / Build	14s
Plugins / Lint (Simple Plugins)	1m 3s
Skit / Test (GPU)	1m 46s
UI / Knip (unused code)	19s
Skit / Test	8m 10s
Plugins / Format Check	15s
All Checks Passed	0s
	48m 23s

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