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>

.gitignore

@@ -0,0 +1,43 @@
+# macOS
+.DS_Store
+
+# Xcode user state / per-user files
+xcuserdata/
+*.xcuserstate
+*.xcuserdatad/
+*.xcscmblueprint
+*.xccheckout
+
+# Xcode build output
+build/
+DerivedData/
+*.moved-aside
+*.hmap
+*.ipa
+*.dSYM.zip
+*.dSYM
+
+# Project-specific local build artifacts (used by Cursor/xcodebuild here)
+.build_artifacts/
+
+# Swift Package Manager
+.build/
+Packages/
+*.xcodeproj/project.xcworkspace/xcuserdata/
+.swiftpm/
+# Note: keep Package.resolved committed for apps (not ignored).
+
+# CocoaPods (only ignore if you don't vendor Pods/)
+# Pods/
+
+# Carthage
+Carthage/Build/
+
+# fastlane
+fastlane/report.xml
+fastlane/Preview.html
+fastlane/screenshots/**/*.png
+fastlane/test_output
+
+# Misc
+*.log