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/tsconfig.src.json

@@ -0,0 +1,15 @@
+{
+  "extends": "@robonen/tsconfig/tsconfig.dom.json",
+  "compilerOptions": {
+    "composite": true,
+    "types": [],
+    "paths": {
+      "@/*": ["./src/*"],
+      "@/composables/*": ["./src/composables/*"],
+      "@/types": ["./src/types"],
+      "@/utils": ["./src/utils"]
+    },
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.src.tsbuildinfo"
+  },
+  "include": ["src/**/*.ts"]
+}