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/api/argocd.ts

@@ -0,0 +1,100 @@
+// --- ArgoCD API types ---
+
+/**
+ * Health status values returned by ArgoCD Application status.
+ */
+export type ArgoCDHealthStatus =
+  | "Healthy"
+  | "Degraded"
+  | "Progressing"
+  | "Missing"
+  | "Unknown";
+
+/**
+ * Sync status values returned by ArgoCD Application status.
+ */
+export type ArgoCDSyncStatus = "Synced" | "OutOfSync" | "Unknown";
+
+/**
+ * An ArgoCD Application resource.
+ * Matches the ArgoCD server API /api/v1/applications response shape.
+ */
+export interface ArgoCDApplication {
+  metadata: {
+    name: string;
+    namespace: string;
+    uid?: string;
+    labels?: Record<string, string>;
+    annotations?: Record<string, string>;
+    creationTimestamp?: string;
+  };
+  spec: {
+    project: string;
+    sourceRepoURL?: string;
+    targetRevision?: string;
+    path?: string;
+    destination?: {
+      server?: string;
+      namespace?: string;
+      name?: string;
+    };
+    sources?: Array<{
+      repoURL?: string;
+      targetRevision?: string;
+      path?: string;
+    }>;
+  };
+  status: {
+    health?: {
+      status: ArgoCDHealthStatus;
+      message?: string;
+    };
+    sync?: {
+      status: ArgoCDSyncStatus;
+      comparedTo?: {
+        destination?: {
+          server?: string;
+          namespace?: string;
+        };
+        source?: {
+          repoURL?: string;
+          path?: string;
+          targetRevision?: string;
+        };
+      };
+    };
+    history?: Array<{
+      dexKey: string; // ISO 8601 timestamp
+      id: number;
+      revision: string;
+      deployStartedAt?: string;
+      triggeredBy?: string;
+    }>;
+    resources?: Array<{
+      kind: string;
+      namespace?: string;
+      name: string;
+      group?: string;
+      status?: string;
+      health?: {
+        status: ArgoCDHealthStatus;
+      };
+    }>;
+    sourceType?: string;
+    summary?: {
+      externalURLs?: string[];
+      images?: string[];
+    };
+  };
+}
+
+/**
+ * Response envelope for the ArgoCD Applications list API.
+ */
+export interface ArgoCDApplicationsList {
+  items: ArgoCDApplication[];
+  metadata?: {
+    resourceVersion?: string;
+    remainingItemCount?: number;
+  };
+}