fix: add eslint as direct devDependency

pnpm's strict node_modules layout does not expose transitive dependency
binaries. eslint was only a transitive dep via @kinvolk/headlamp-plugin,
causing 'eslint: not found' when running pnpm run lint in CI.

Adding eslint@^8.57.0 as a direct devDependency ensures the binary is
available in node_modules/.bin/ under pnpm.

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

package.json

@@ -47,6 +47,7 @@
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@vitest/coverage-v8": "^3.2.4",
+    "eslint": "^8.57.0",
     "jsdom": "^24.0.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",