platos: segundo plato por comida (ensalada + salmon)

- MealSlot.secondaryDishId (opcional, aditivo CloudKit); se limpia al
  sustituir/quitar/vaciar/marcar comer fuera y viaja en swaps, copia de
  semana anterior, undo y snapshot KV de iCloud (retrocompatible)
- Picker de hueco lleno: toggle "anadir como segundo plato" + boton para
  quitarlo; HomeViewModel.assignSecondaryDish/removeSecondaryDish
- Se muestra "Plato + Segundo" en calendario, widget y export; la lista
  de la compra incluye los ingredientes del segundo; stats lo cuentan
- Evento analytics secondary_dish_added; strings en 6 idiomas

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013H6bXqGX1ygwib1Dm3n3UG

MealMood/Models/MealSlot.swift

@@ -8,6 +8,9 @@ final class MealSlot {
     var dayOfWeek: Int = 0 // 0=Monday, 6=Sunday
     var mealType: String = MealType.dinner.rawValue
     var dishId: UUID?
+    /// Optional companion dish ("ensalada + salmón"). Only meaningful while
+    /// `dishId` is set; cleared together with it.
+    var secondaryDishId: UUID?
     var calendarEventId: String?
     var isRuleOverridden: Bool = false
     var isEatingOut: Bool = false
@@ -19,6 +22,7 @@ final class MealSlot {
         dayOfWeek: Int,
         mealType: String,
         dishId: UUID? = nil,
+        secondaryDishId: UUID? = nil,
         calendarEventId: String? = nil,
         isRuleOverridden: Bool = false,
         isEatingOut: Bool = false
@@ -27,6 +31,7 @@ final class MealSlot {
         self.dayOfWeek = dayOfWeek
         self.mealType = mealType
         self.dishId = dishId
+        self.secondaryDishId = secondaryDishId
         self.calendarEventId = calendarEventId
         self.isRuleOverridden = isRuleOverridden
         self.isEatingOut = isEatingOut