Add cross-device grocery list sync via Nextcloud Cookbook API

Store a _groceryState JSON field on each recipe to track which
ingredients have been added, completed, or removed. Uses per-item
last-writer-wins conflict resolution with ISO 8601 timestamps.
Debounced push (2s) avoids excessive API calls; pull reconciles
on recipe open and app launch. Includes a settings toggle to
enable/disable sync.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Nextcloud Cookbook iOS Client/Views/MainView.swift

@@ -81,6 +81,10 @@ struct MainView: View {
                 }
             }
             await groceryList.load()
+            groceryList.configureSyncManager(appState: appState)
+            if UserSettings.shared.grocerySyncEnabled {
+                await groceryList.syncManager?.performInitialSync()
+            }
             recipeViewModel.presentLoadingIndicator = false
         }
     }