test: add component test coverage for all untested files

Adds 60 new tests (108 total) covering every untested module:
- IntelGpuDataContext: provider renders, loading/loaded states, CRD
  available/unavailable paths, refresh, useIntelGpuContext throws outside
  provider
- OverviewPage: loading, plugin-not-detected, error, populated, refresh
  button, CRD notice, device plugin table, plugin daemon pods, active pods
- NodesPage: loading, empty state, GPU node summary table, detail cards
- PodsPage: loading, empty state, summary counts, pending pod attention,
  all-pods table
- DevicePluginsPage: loading, CRD unavailable, no-plugins, plugin detail,
  daemon pod table
- NodeDetailSection: null for non-GPU nodes, GPU capacity/allocatable rows,
  pod list, loading state
- PodDetailSection: null for non-GPU pods, GPU resource rows, phase status,
  limits-only containers
- MetricsPage: context loading gate, Prometheus unreachable, empty chips,
  chip cards with power values, MetricRequirements always rendered, refresh

Also fixes vitest.config.mts to pin NODE_ENV=test so tests run correctly
without requiring callers to set it explicitly.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

vitest.config.mts

@@ -6,5 +6,8 @@ export default defineConfig({
     environment: 'jsdom',
     setupFiles: ['./vitest.setup.ts'],
     exclude: ['e2e/**', 'node_modules/**'],
+    env: {
+      NODE_ENV: 'test',
+    },
   },
 });