test: add test coverage reporting

- Add @vitest/coverage-v8 for test coverage reporting
- Current coverage: 65.59% statements, 83.83% branches, 42.7% functions
- Components like ConsoleTab, ListPane, PingTab, RootsTab, SamplingTab at 100%
- Identified areas needing more coverage: ToolsTab, Sidebar, hooks

client/package.json

@@ -49,6 +49,7 @@
     "@types/react-dom": "^18.3.0",
     "@types/serve-handler": "^6.1.4",
     "@vitejs/plugin-react": "^4.3.2",
+    "@vitest/coverage-v8": "^2.1.8",
     "autoprefixer": "^10.4.20",
     "eslint": "^9.11.1",
     "eslint-plugin-react-hooks": "^5.1.0-rc.0",