Add Share Extension for importing recipes via URL

Adds a Share Extension so users can share URLs from Safari (or any app)
to open the main app with the ImportURLSheet pre-filled. Uses a custom
URL scheme (nextcloud-cookbook://) as the bridge between the extension
and the main app.

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

Nextcloud Cookbook iOS Client/Views/Recipes/ImportURLSheet.swift

@@ -10,6 +10,7 @@ struct ImportURLSheet: View {
     @Environment(\.dismiss) private var dismiss
 
     var onImport: (RecipeDetail) -> Void
+    var initialURL: String = ""
 
     @State private var url: String = ""
     @State private var isLoading: Bool = false
@@ -62,6 +63,11 @@ struct ImportURLSheet: View {
             } message: {
                 Text(alertMessage)
             }
+            .onAppear {
+                if !initialURL.isEmpty {
+                    url = initialURL
+                }
+            }
         }
     }