feat(tests): add basic arithmetic functions and corresponding tests

- Implemented `add` and `subtract` functions in multiple packages (web, ui, utils).
- Created unit tests for these functions using Vitest in respective test files.
- Updated turbo.json to include a new test pipeline for coverage and watch mode.

apps/web/package.json

@@ -7,7 +7,9 @@
     "dev": "vite --clearScreen false",
     "build": "tsc && vite build",
     "preview": "vite preview",
-    "lint": "eslint \"src/**/*.ts\""
+    "lint": "eslint \"src/**/*.ts\"",
+    "test": "vitest run",
+    "test:watch": "vitest --watch"
   },
   "dependencies": {
     "@repo/ui": "workspace:*",
@@ -26,6 +28,7 @@
     "@vitejs/plugin-react": "^5.1.2",
     "eslint": "^8.57.1",
     "typescript": "5.5.4",
-    "vite": "^5.1.4"
+    "vite": "^5.1.4",
+    "vitest": "^4.0.17"
   }
 }