feat(ApplicationsList): implement ArgoCD Applications List view

Implement the Applications List view for headlamp-argocd-plugin (PRI-189).

- Add src/components/ApplicationsList.tsx with table of all ArgoCD Applications
  showing: app name, namespace, project, health status, sync status,
  target revision, and last synced time
- Health/sync status badges using ArgoCD color conventions
- Filter controls: health dropdown, sync dropdown, project dropdown
- Friendly "ArgoCD not detected" error state when ArgoCD is unreachable
- Add src/api/argocd.ts with ArgoCD API types (Application, ApplicationsList)
- Add unit tests in src/__tests__/ApplicationsList.test.tsx:
  - Pure function tests for healthStatusToColor and syncStatusToColor
  - Filter logic unit tests
  - Component smoke tests (loading, error, data, empty states)
- Replace stub view in src/index.tsx with ApplicationsList component

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

src/index.test.tsx

@@ -1,4 +1,4 @@
-import { describe, it, expect } from "vitest";
+import { describe, expect, it } from "vitest";
 
 // Minimal smoke test — verify the test file itself is valid and can run.
 // Full plugin component tests will be added in subsequent tasks per PRI-189.