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>

tests/__mocks__/imports.ts

@@ -0,0 +1,35 @@
+/**
+ * Minimal mock for Nuxt's #imports auto-import.
+ * Add stubs here as tests require them.
+ */
+
+import { ref } from 'vue'
+
+export const useRuntimeConfig = () => ({
+  public: {
+    apiBaseUrl: 'http://localhost:8081/api',
+    appVersion: '0.0.0-test',
+  },
+})
+
+export const useRoute = () => ({
+  path: '/',
+  params: {},
+  query: {},
+})
+
+export const useRouter = () => ({
+  push: () => Promise.resolve(),
+  replace: () => Promise.resolve(),
+})
+
+export const navigateTo = () => Promise.resolve()
+
+export const useRequestFetch = () => fetch
+
+export const useFetch = () => ({
+  data: ref(null),
+  error: ref(null),
+  pending: ref(false),
+  refresh: () => Promise.resolve(),
+})