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>

vitest.config.ts

@@ -0,0 +1,17 @@
+import { defineConfig } from 'vitest/config'
+import { resolve } from 'node:path'
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'happy-dom',
+    root: '.',
+    include: ['tests/**/*.test.ts'],
+  },
+  resolve: {
+    alias: {
+      '~': resolve(__dirname, 'app'),
+      '#imports': resolve(__dirname, 'tests/__mocks__/imports.ts'),
+    },
+  },
+})