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/eslint.config.ts

@@ -0,0 +1,3 @@
+import { base, compose, imports, stylistic, typescript, vitest, vue } from '@robonen/eslint';
+
+export default compose(base, typescript, vue, vitest, imports, stylistic);