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/StarFieldView.swift

@@ -0,0 +1,92 @@
+//
+//  StarFieldView.swift
+//  MoonWatch
+//
+
+import SwiftUI
+
+struct StarFieldView: View {
+    var starCount: Int = 160
+
+    @State private var stars: [Star] = []
+
+    private struct Star: Identifiable {
+        let id = UUID()
+        let position: CGPoint   // 0...1 normalized
+        let radius: CGFloat
+        let baseOpacity: Double
+        let twinklePhase: Double
+        let twinkleSpeed: Double
+        let warm: Bool
+    }
+
+    var body: some View {
+        GeometryReader { geo in
+            TimelineView(.animation(minimumInterval: 1.0 / 12.0)) { ctx in
+                Canvas { context, size in
+                    let t = ctx.date.timeIntervalSinceReferenceDate
+                    for star in stars {
+                        let twinkle = (sin(t * star.twinkleSpeed + star.twinklePhase) + 1) / 2
+                        let alpha = star.baseOpacity * (0.35 + 0.65 * twinkle)
+                        let center = CGPoint(
+                            x: star.position.x * size.width,
+                            y: star.position.y * size.height
+                        )
+                        let rect = CGRect(
+                            x: center.x - star.radius,
+                            y: center.y - star.radius,
+                            width: star.radius * 2,
+                            height: star.radius * 2
+                        )
+                        let color: Color = star.warm
+                            ? Color(red: 1.0, green: 0.92, blue: 0.78).opacity(alpha)
+                            : Color(red: 0.92, green: 0.95, blue: 1.0).opacity(alpha)
+                        context.fill(Path(ellipseIn: rect), with: .color(color))
+                        if star.radius > 1.1 {
+                            // Soft halo for the larger stars.
+                            let halo = rect.insetBy(dx: -star.radius * 1.6, dy: -star.radius * 1.6)
+                            context.fill(
+                                Path(ellipseIn: halo),
+                                with: .color(color.opacity(0.20))
+                            )
+                        }
+                    }
+                }
+            }
+            .onAppear {
+                if stars.isEmpty {
+                    stars = Self.generate(count: starCount)
+                }
+            }
+        }
+        .ignoresSafeArea()
+        .allowsHitTesting(false)
+    }
+
+    private static func generate(count: Int) -> [Star] {
+        var rng = SystemRandomNumberGenerator()
+        return (0..<count).map { _ in
+            let big = Double.random(in: 0...1, using: &rng) > 0.88
+            return Star(
+                position: CGPoint(
+                    x: CGFloat.random(in: 0...1, using: &rng),
+                    y: CGFloat.random(in: 0...1, using: &rng)
+                ),
+                radius: big
+                    ? CGFloat.random(in: 1.4...2.2, using: &rng)
+                    : CGFloat.random(in: 0.4...1.1, using: &rng),
+                baseOpacity: Double.random(in: 0.35...0.95, using: &rng),
+                twinklePhase: Double.random(in: 0...(2 * .pi), using: &rng),
+                twinkleSpeed: Double.random(in: 0.5...1.7, using: &rng),
+                warm: Double.random(in: 0...1, using: &rng) > 0.75
+            )
+        }
+    }
+}
+
+#Preview {
+    ZStack {
+        Color(red: 0.03, green: 0.04, blue: 0.10).ignoresSafeArea()
+        StarFieldView()
+    }
+}