feat: add Reachability Center and Why Drawer components with tests

- Implemented ReachabilityCenterComponent for displaying asset reachability status with summary and filtering options.
- Added ReachabilityWhyDrawerComponent to show detailed reachability evidence and call paths.
- Created unit tests for both components to ensure functionality and correctness.
- Updated accessibility test results for the new components.

docs/api/gateway/samples/vex-statement-detail.json

@@ -0,0 +1,18 @@
+{
+  "statementId": "vex::tenant-default::CVE-2024-12345::001",
+  "vulnId": "CVE-2024-12345",
+  "productId": "asset::registry.local/ops/auth",
+  "status": "not_affected",
+  "justification": "Component not present in runtime image.",
+  "issuer": "vendor:example",
+  "affectedPurls": ["pkg:npm/example@1.2.3"],
+  "references": [
+    {
+      "label": "Analysis report",
+      "url": "https://example.invalid/vex/report/001"
+    }
+  ],
+  "updatedAt": "2025-12-10T00:00:00Z",
+  "etag": "\"vex-001-v1\""
+}
+