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/Data/GroceryStateModels.swift

@@ -0,0 +1,58 @@
+//
+//  GroceryStateModels.swift
+//  Nextcloud Cookbook iOS Client
+//
+
+import Foundation
+
+/// Tracks grocery list state for a recipe, stored as `_groceryState` in the recipe JSON on the server.
+struct GroceryState: Codable {
+    var version: Int = 1
+    var lastModified: String
+    var items: [String: GroceryItemState]
+
+    init(lastModified: String = GroceryStateDate.now(), items: [String: GroceryItemState] = [:]) {
+        self.version = 1
+        self.lastModified = lastModified
+        self.items = items
+    }
+}
+
+struct GroceryItemState: Codable {
+    enum Status: String, Codable {
+        case added
+        case completed
+        case removed
+    }
+
+    var status: Status
+    var addedAt: String
+    var modifiedAt: String
+
+    init(status: Status, addedAt: String = GroceryStateDate.now(), modifiedAt: String = GroceryStateDate.now()) {
+        self.status = status
+        self.addedAt = addedAt
+        self.modifiedAt = modifiedAt
+    }
+}
+
+/// ISO 8601 date helpers. Dates are stored as strings to avoid coupling to a parent encoder's date strategy.
+enum GroceryStateDate {
+    private static let formatter: ISO8601DateFormatter = {
+        let f = ISO8601DateFormatter()
+        f.formatOptions = [.withInternetDateTime]
+        return f
+    }()
+
+    static func now() -> String {
+        formatter.string(from: Date())
+    }
+
+    static func date(from string: String) -> Date? {
+        formatter.date(from: string)
+    }
+
+    static func string(from date: Date) -> String {
+        formatter.string(from: date)
+    }
+}