fix(vue): eslint/tsconfig migration + resolve type errors

@robonen/vue (toolkit): migrate to eslint flat config + composite tsconfig;
fix composable + test type errors (writable computed returns, null guards,
overload-compatible signatures, typed test helpers) — all type-level.

vue/toolkit/src/composables/state/index.ts

@@ -2,5 +2,7 @@ export * from './useAppSharedState';
 export * from './useAsyncState';
 export * from './useContextFactory';
 export * from './useCounter';
+export * from './useId';
 export * from './useInjectionStore';
+export * from './useStepper';
 export * from './useToggle';