feat: tap widget → opens app to food search

Widget: .widgetURL(platform://add-food) on all widget sizes
App: .onOpenURL handles platform://add-food → switches to Fitness
tab and opens the food assistant sheet

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/ContentView.swift

@@ -120,6 +120,12 @@ struct MainTabView: View {
         .sheet(isPresented: $showAssistant) {
             AssistantSheetView(onFoodAdded: foodAdded)
         }
+        .onOpenURL { url in
+            if url.scheme == "platform" && url.host == "add-food" {
+                selectedTab = 1 // Fitness tab
+                showAssistant = true
+            }
+        }
         .task {
             guard showReader else { return }
             let renderer = ArticleRenderer.shared