fix(primitives): eslint/tsconfig migration, asChild refactor, type fixes

- Migrate to eslint flat config + composite tsconfig.
- Complete the asChild→as="template" refactor (remove asChild prop + :as-child
  bindings across components, matching Primitive's slot model).
- Fix test type errors and source type-safety (useGraceArea hull/point math,
  FocusScope/util ref typing).

Note: ~53 vue-tsc errors remain (HTML attr/event passthrough typing on
transparent wrapper components + a couple of duplicate-export naming
collisions) — not gated by CI (build/lint/test green); pending a
component-attribute-typing design decision.

vue/primitives/vitest.config.ts

@@ -1,6 +1,7 @@
+import { resolve } from 'node:path';
+import { playwright } from '@vitest/browser-playwright';
 import Vue from 'unplugin-vue/vite';
 import { defineConfig } from 'vitest/config';
-import { resolve } from 'node:path';
 
 export default defineConfig({
   plugins: [Vue()],
@@ -13,6 +14,11 @@ export default defineConfig({
     },
   },
   test: {
-    environment: 'jsdom',
+    browser: {
+      enabled: true,
+      provider: playwright(),
+      headless: true,
+      instances: [{ browser: 'chromium' }],
+    },
   },
 });