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/stores/uiStore.ts

@@ -43,6 +43,11 @@ interface UIState {
 	createThreadModalOpen: boolean;
 	openCreateThreadModal: () => void;
 	closeCreateThreadModal: () => void;
+
+	// External link dialog
+	externalLinkUrl: string | null;
+	openExternalLink: (url: string) => void;
+	closeExternalLink: () => void;
 }
 
 export const useUIStore = create<UIState>((set) => ({
@@ -93,4 +98,9 @@ export const useUIStore = create<UIState>((set) => ({
 	createThreadModalOpen: false,
 	openCreateThreadModal: () => set({ createThreadModalOpen: true }),
 	closeCreateThreadModal: () => set({ createThreadModalOpen: false }),
+
+	// External link dialog
+	externalLinkUrl: null,
+	openExternalLink: (url) => set({ externalLinkUrl: url }),
+	closeExternalLink: () => set({ externalLinkUrl: null }),
 }));