fix: add explicit dev dependencies and fix React test environment

- Add react, react-dom, vitest, jsdom, and testing-library as explicit
  devDependencies instead of relying on transitive deps from
  @kinvolk/headlamp-plugin
- Add peerDependencies for react/react-dom
- Set process.env.NODE_ENV to "test" in vitest config to prevent React
  from loading its production build (which blocks act())
- Do NOT include canvas as a dependency — it requires native build tools
  (pangocairo, etc.) not present in the CI node:22 container

Fixes CI install failures from prior PR #10 which included canvas.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -24,7 +24,18 @@
     "test": "vitest run",
     "test:watch": "vitest"
   },
+  "peerDependencies": {
+    "react": "^18.0.0",
+    "react-dom": "^18.0.0"
+  },
   "devDependencies": {
-    "@kinvolk/headlamp-plugin": "^0.13.0"
+    "@kinvolk/headlamp-plugin": "^0.13.0",
+    "@testing-library/jest-dom": "^6.4.8",
+    "@testing-library/react": "^16.0.0",
+    "@testing-library/user-event": "^14.5.2",
+    "jsdom": "^24.0.0",
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1",
+    "vitest": "^3.0.5"
   }
 }