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/PlatformWidget/PlatformWidget.swift

@@ -290,6 +290,7 @@ struct PlatformWidget: Widget {
     var body: some WidgetConfiguration {
         StaticConfiguration(kind: kind, provider: CalorieProvider()) { entry in
             PlatformWidgetEntryView(entry: entry)
+                .widgetURL(URL(string: "platform://add-food"))
                 .containerBackground(.fill.tertiary, for: .widget)
         }
         .configurationDisplayName("Calories")