feat(macos): add native menu bar app with notch overlay

Introduces a SwiftUI companion app for the Fail Well PWA: menu bar
status item, popover with task list / record / history views, command
palette, and a Dynamic Island-style notch overlay that shows the active
step and timer beside the hardware notch and expands on hover to reveal
pause/next controls.

macos/Sources/FailWell/Models/TimeRange.swift

@@ -0,0 +1,11 @@
+import Foundation
+
+struct TimeRange: Codable, Hashable, Sendable {
+    var start: Date
+    var end: Date?
+
+    var duration: TimeInterval? {
+        guard let end else { return nil }
+        return end.timeIntervalSince(start)
+    }
+}