Add meal plan feature with cross-device sync and automatic stale data cleanup

Introduces weekly meal planning with a calendar-based tab view, per-recipe
date assignments synced via Nextcloud Cookbook custom metadata, and 30-day
automatic pruning of old entries on load, save, and sync merge.

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

Nextcloud Cookbook iOS Client/Data/ObservableRecipeDetail.swift

@@ -27,6 +27,7 @@ class ObservableRecipeDetail: ObservableObject {
     @Published var recipeInstructions: [String]
     @Published var nutrition: [String:String]
     var groceryState: GroceryState?
+    var mealPlanAssignment: MealPlanAssignment?
 
     // Additional functionality
     @Published var ingredientMultiplier: Double
@@ -50,6 +51,7 @@ class ObservableRecipeDetail: ObservableObject {
         recipeInstructions = []
         nutrition = [:]
         groceryState = nil
+        mealPlanAssignment = nil
 
         ingredientMultiplier = 1
     }
@@ -71,6 +73,7 @@ class ObservableRecipeDetail: ObservableObject {
         recipeInstructions = recipeDetail.recipeInstructions
         nutrition = recipeDetail.nutrition
         groceryState = recipeDetail.groceryState
+        mealPlanAssignment = recipeDetail.mealPlanAssignment
 
         ingredientMultiplier = Double(recipeDetail.recipeYield == 0 ? 1 : recipeDetail.recipeYield)
     }
@@ -94,7 +97,8 @@ class ObservableRecipeDetail: ObservableObject {
             recipeIngredient: self.recipeIngredient,
             recipeInstructions: self.recipeInstructions,
             nutrition: self.nutrition,
-            groceryState: self.groceryState
+            groceryState: self.groceryState,
+            mealPlanAssignment: self.mealPlanAssignment
         )
     }