feat(09-01): configure Vitest with multi-project setup

- Install Vitest, @vitest/browser, vitest-browser-svelte, @vitest/coverage-v8
- Configure multi-project: client (browser/Playwright) and server (node)
- Add test scripts: test, test:unit, test:unit:watch, test:coverage
- Coverage provider: v8 with autoUpdate thresholds

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

vite.config.ts

@@ -1,7 +1,44 @@
 import { sveltekit } from '@sveltejs/kit/vite';
 import tailwindcss from '@tailwindcss/vite';
+import { playwright } from '@vitest/browser-playwright';
 import { defineConfig } from 'vite';
 
 export default defineConfig({
-	plugins: [tailwindcss(), sveltekit()]
+	plugins: [tailwindcss(), sveltekit()],
+	test: {
+		coverage: {
+			provider: 'v8',
+			reporter: ['text', 'json', 'html'],
+			include: ['src/**/*.{ts,svelte}'],
+			exclude: ['src/**/*.test.ts', 'src/**/*.spec.ts'],
+			thresholds: {
+				autoUpdate: true
+			}
+		},
+		projects: [
+			{
+				extends: true,
+				test: {
+					name: 'client',
+					testTimeout: 5000,
+					browser: {
+						enabled: true,
+						provider: playwright(),
+						instances: [{ browser: 'chromium' }]
+					},
+					include: ['src/**/*.svelte.{test,spec}.{js,ts}'],
+					setupFiles: ['./vitest-setup-client.ts']
+				}
+			},
+			{
+				extends: true,
+				test: {
+					name: 'server',
+					environment: 'node',
+					include: ['src/**/*.{test,spec}.{js,ts}'],
+					exclude: ['src/**/*.svelte.{test,spec}.{js,ts}']
+				}
+			}
+		]
+	}
 });