feat: add external link confirmation dialog for product URLs

Show an external link icon on ItemCard and CandidateCard that opens a
confirmation dialog before navigating to product URLs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/routes/__root.tsx

@@ -11,6 +11,7 @@ import { SlideOutPanel } from "../components/SlideOutPanel";
 import { ItemForm } from "../components/ItemForm";
 import { CandidateForm } from "../components/CandidateForm";
 import { ConfirmDialog } from "../components/ConfirmDialog";
+import { ExternalLinkDialog } from "../components/ExternalLinkDialog";
 import { OnboardingWizard } from "../components/OnboardingWizard";
 import { useUIStore } from "../stores/uiStore";
 import { useOnboardingComplete } from "../hooks/useSettings";
@@ -142,6 +143,9 @@ function RootLayout() {
       {/* Item Confirm Delete Dialog */}
       <ConfirmDialog />
 
+      {/* External Link Confirmation Dialog */}
+      <ExternalLinkDialog />
+
       {/* Candidate Delete Confirm Dialog */}
       {confirmDeleteCandidateId != null && currentThreadId != null && (
         <CandidateDeleteDialog