feat: public item detail view for shared and public setups

Items in shared/public setups are now viewable without auth. Clicking
an item in a shared setup navigates to /items/:id?setup=:setupId&share=token
which fetches the item via a public endpoint authorized by the setup's
visibility or share token. Read-only mode hides all owner controls.

- Added getSetupItemById service function
- Added GET /api/shared/:token/items/:itemId endpoint
- Added GET /api/setups/:setupId/items/:itemId/public endpoint
- Added usePublicSetupItem and useSharedSetupItem hooks
- Item detail page detects setup context and switches to public fetch
- Back link returns to setup instead of collection in setup context

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

src/client/hooks/useSetups.ts

@@ -83,6 +83,51 @@ export function useSharedSetup(token: string | null) {
 	});
 }
 
+interface SetupItem {
+	id: number;
+	name: string;
+	brand?: string | null;
+	weightGrams: number | null;
+	priceCents: number | null;
+	quantity: number;
+	categoryId: number;
+	notes: string | null;
+	productUrl: string | null;
+	imageFilename: string | null;
+	imageUrl?: string | null;
+	globalItemId: number | null;
+	createdAt: string;
+	categoryName: string;
+	categoryIcon: string;
+	classification: string;
+}
+
+export function usePublicSetupItem(
+	setupId: number | null,
+	itemId: number | null,
+) {
+	return useQuery({
+		queryKey: ["setups", setupId, "items", itemId, "public"],
+		queryFn: () =>
+			apiGet<SetupItem>(`/api/setups/${setupId}/items/${itemId}/public`),
+		enabled: setupId != null && itemId != null,
+		retry: (count, error) =>
+			error instanceof ApiError && error.status === 404 ? false : count < 3,
+	});
+}
+
+export function useSharedSetupItem(
+	token: string | null,
+	itemId: number | null,
+) {
+	return useQuery({
+		queryKey: ["shared-setup", token, "items", itemId],
+		queryFn: () => apiGet<SetupItem>(`/api/shared/${token}/items/${itemId}`),
+		enabled: !!token && itemId != null,
+		retry: false,
+	});
+}
+
 export function useCreateSetup() {
 	const queryClient = useQueryClient();
 	return useMutation({