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/GroceryListMode.swift

@@ -0,0 +1,20 @@
+//
+//  GroceryListMode.swift
+//  Nextcloud Cookbook iOS Client
+//
+
+import Foundation
+
+enum GroceryListMode: String, CaseIterable {
+    case inApp = "inApp"
+    case appleReminders = "appleReminders"
+
+    func descriptor() -> String {
+        switch self {
+        case .inApp: return String(localized: "In-App")
+        case .appleReminders: return String(localized: "Apple Reminders")
+        }
+    }
+
+    static let allValues: [GroceryListMode] = GroceryListMode.allCases
+}