feat: add Playwright E2E smoke tests and fix empty namespace crash

Fix getNamespaces() to skip cluster-scoped resources (Namespace: "")
that caused Router.createRouteURL to throw TypeError on the Namespaces
page. Add Playwright E2E smoke tests with Authentik OIDC auth for CI
and K8s token fallback for local dev. Add Gitea Actions E2E workflow,
vitest unit test infrastructure, and test-utils fixtures.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -12,9 +12,12 @@
     "format": "prettier --write src/",
     "format:check": "prettier --check src/",
     "test": "vitest run",
-    "test:watch": "vitest"
+    "test:watch": "vitest",
+    "e2e": "playwright test",
+    "e2e:headed": "playwright test --headed"
   },
   "devDependencies": {
-    "@kinvolk/headlamp-plugin": "^0.13.0"
+    "@kinvolk/headlamp-plugin": "^0.13.0",
+    "@playwright/test": "^1.58.2"
   }
 }