chore: add Playwright test scaffolding

Adds @playwright/test and @types/node as dev dependencies, playwright.config.ts
with a localhost:1420 base URL targeting the Vite dev server, an example
spec, and the standard Playwright output directories to .gitignore.

package.json

@@ -37,9 +37,11 @@
     "zustand": "^5.0.12"
   },
   "devDependencies": {
+    "@playwright/test": "^1.61.1",
     "@tailwindcss/vite": "^4.2.2",
     "@tauri-apps/cli": "^2",
     "@types/d3-force": "^3.0.10",
+    "@types/node": "^26.0.1",
     "@types/react": "^19.1.8",
     "@types/react-dom": "^19.1.6",
     "@vitejs/plugin-react": "^4.6.0",