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/package.json

@@ -37,19 +37,18 @@
     }
   },
   "scripts": {
-    "lint:check": "oxlint -c oxlint.config.ts",
-    "lint:fix": "oxlint -c oxlint.config.ts --fix",
+    "lint:check": "eslint .",
+    "lint:fix": "eslint . --fix",
     "test": "vitest run",
     "dev": "vitest dev",
     "build": "tsdown"
   },
   "devDependencies": {
-    "@robonen/oxlint": "workspace:*",
+    "@robonen/eslint": "workspace:*",
     "@robonen/tsconfig": "workspace:*",
     "@robonen/tsdown": "workspace:*",
-    "@stylistic/eslint-plugin": "catalog:",
     "@vue/test-utils": "catalog:",
-    "oxlint": "catalog:",
+    "eslint": "catalog:",
     "tsdown": "catalog:"
   },
   "dependencies": {