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.

turbo.json

@@ -29,6 +29,22 @@
     "dev": {
       "cache": false,
       "persistent": true
+    },
+    "test": {
+      "dependsOn": [
+        "^test"
+      ],
+      "inputs": [
+        "$TURBO_DEFAULT$"
+      ],
+      "outputs": [
+        "coverage/**",
+        ".vitest/**"
+      ]
+    },
+    "test:watch": {
+      "cache": false,
+      "persistent": true
     }
   }
 }