feat: add ExemptionManager tests, coverage threshold, and ArtifactHub metadata

- Add 22 unit tests for ExemptionManager.tsx covering:
  - Failing checks extraction (pod-level, container-level, ignore-severity, dedup)
  - Dialog open/close, check toggle, exempt-all toggle
  - Apply button enabled/disabled state
  - ApiProxy.request called with correct path (apps/batch/core) and annotation structure
  - Success and error feedback states, in-flight "Applying..." label
- Add vitest coverage config with >=80% threshold (lines/functions/branches/statements)
- Update artifacthub-pkg.yml:
  - Add install section (Headlamp-native plugin installer only)
  - Add appVersion: "5.0" (compatible Polaris dashboard version)
  - Expand distro-compat from "in-cluster" to "in-cluster,web,desktop"
  - Add changes block documenting v1.0 features

Closes privilegedescalation/headlamp-polaris-plugin#81 (partial — test and metadata tasks)

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

vitest.config.mts

@@ -9,5 +9,16 @@ export default defineConfig({
     environment: 'jsdom',
     setupFiles: ['./vitest.setup.ts'],
     exclude: ['e2e/**', 'node_modules/**'],
+    coverage: {
+      provider: 'v8',
+      include: ['src/**/*.{ts,tsx}'],
+      exclude: ['src/**/*.test.{ts,tsx}', 'src/test-utils.tsx', 'src/index.tsx'],
+      thresholds: {
+        lines: 80,
+        functions: 80,
+        branches: 80,
+        statements: 80,
+      },
+    },
   },
 });