Add MoonWatch app, widget extension, shared module, and .gitignore

Adds the real MoonWatch implementation on top of the initial Xcode
template: new SwiftUI views (MoonDiskView, StarFieldView), the
MoonWatchWidget extension, and a Shared module containing the moon
phase calculator, shadow shape, and shared asset catalog. Also adds a
Swift/Xcode/macOS .gitignore and untracks per-user xcuserdata files.

Co-authored-by: Cursor <cursoragent@cursor.com>

MoonWatch/MoonWatchApp.swift

@@ -12,6 +12,7 @@ struct MoonWatchApp: App {
     var body: some Scene {
         WindowGroup {
             ContentView()
+                .preferredColorScheme(.dark)
         }
     }
 }