Add Apple Reminders integration for grocery list with local mapping persistence

Introduce a GroceryListManager facade that delegates to either the existing
in-app GroceryList or a new RemindersGroceryStore backed by EventKit. Users
choose the mode in Settings; when Reminders mode is active the Grocery List
tab is hidden. Recipe-to-reminder grouping uses a local mapping file
(reminder_mappings.data) instead of polluting the reminder's notes field,
with automatic pruning when reminders are deleted externally.

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

Nextcloud Cookbook iOS Client/Data/UserSettings.swift

@@ -120,6 +120,18 @@ class UserSettings: ObservableObject {
             UserDefaults.standard.set(decimalNumberSeparator, forKey: "decimalNumberSeparator")
         }
     }
+
+    @Published var groceryListMode: String {
+        didSet {
+            UserDefaults.standard.set(groceryListMode, forKey: "groceryListMode")
+        }
+    }
+
+    @Published var remindersListIdentifier: String {
+        didSet {
+            UserDefaults.standard.set(remindersListIdentifier, forKey: "remindersListIdentifier")
+        }
+    }
     
     init() {
         self.username = UserDefaults.standard.object(forKey: "username") as? String ?? ""
@@ -140,6 +152,8 @@ class UserSettings: ObservableObject {
         self.expandInfoSection = UserDefaults.standard.object(forKey: "expandInfoSection") as? Bool ?? false
         self.keepScreenAwake = UserDefaults.standard.object(forKey: "keepScreenAwake") as? Bool ?? true
         self.decimalNumberSeparator = UserDefaults.standard.object(forKey: "decimalNumberSeparator") as? String ?? "."
+        self.groceryListMode = UserDefaults.standard.object(forKey: "groceryListMode") as? String ?? GroceryListMode.inApp.rawValue
+        self.remindersListIdentifier = UserDefaults.standard.object(forKey: "remindersListIdentifier") as? String ?? ""
         
         if authString == "" {
             if token != "" && username != "" {