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-evidence-list.json

@@ -0,0 +1,16 @@
+{
+  "statementId": "vex::tenant-default::CVE-2024-12345::001",
+  "items": [
+    {
+      "evidenceId": "evidence::001",
+      "kind": "document",
+      "url": "https://example.invalid/evidence/001.json?sig=mock",
+      "sha256": "sha256:0000000000000000000000000000000000000000000000000000000000000000",
+      "expiresAt": "2025-12-31T23:59:59Z"
+    }
+  ],
+  "count": 1,
+  "etag": "\"vex-evidence-sample-v1\"",
+  "traceId": "trace-sample-3"
+}
+