test: configure Vitest and add 54 unit tests (F6.1, F6.2)

Set up Vitest with happy-dom, mock Nuxt auto-imports via #imports alias.
Add tests for: inventory-types validators (9), apiHelpers (10),
modelUtils (18), useConfirm (8), useToast (9). All 54 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -10,7 +10,9 @@
     "postinstall": "nuxt prepare",
     "start": "nuxt start",
     "lint": "eslint . --ext .js,.ts,.vue",
-    "lint:fix": "npm run lint -- --fix"
+    "lint:fix": "npm run lint -- --fix",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@nuxtjs/tailwindcss": "^6.14.0",
@@ -28,10 +30,13 @@
     "@types/node": "^25.2.1",
     "@typescript-eslint/eslint-plugin": "^8.44.1",
     "@typescript-eslint/parser": "^8.44.1",
+    "@vue/test-utils": "^2.4.6",
     "eslint": "^9.36.0",
     "eslint-plugin-vue": "^10.5.0",
+    "happy-dom": "^20.5.3",
     "typescript": "^5.7.3",
     "unplugin-icons": "^0.19.3",
+    "vitest": "^4.0.18",
     "vue-eslint-parser": "^10.2.0"
   }
 }