fix: frontend error handling — ErrorBoundary, type-safe errors, stable keys

- React ErrorBoundary wrapping entire app for graceful crash recovery
- fetchJSON error handling uses try/catch instead of .catch() chain
- CertificateDetailPage: instanceof checks replace unsafe type casts
- DataTable: keyField prop replaces array index keys

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/api/client.ts

@@ -33,8 +33,14 @@ async function fetchJSON<T>(url: string, init?: RequestInit): Promise<T> {
     throw new Error('Authentication required');
   }
   if (!res.ok) {
-    const body = await res.json().catch(() => ({ message: res.statusText }));
-    throw new Error(body.message || body.error || `HTTP ${res.status}`);
+    let errorMsg = res.statusText;
+    try {
+      const body = await res.json();
+      errorMsg = body.message || body.error || errorMsg;
+    } catch {
+      // Response body is not JSON, use status text
+    }
+    throw new Error(errorMsg || `HTTP ${res.status}`);
   }
   return res.json();
 }