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/Views/Router.swift

@@ -0,0 +1,37 @@
+import Foundation
+import Observation
+
+@MainActor
+@Observable
+final class Router {
+    enum Screen: Hashable, Sendable {
+        case list
+        case detail(taskId: String)
+        case editor(taskId: String?)
+        case record(taskId: String)
+        case history(taskId: String)
+        case settings
+    }
+
+    var stack: [Screen] = [.list]
+    var showPalette: Bool = false
+
+    var current: Screen { stack.last ?? .list }
+
+    func push(_ screen: Screen) {
+        stack.append(screen)
+    }
+
+    func replace(with screen: Screen) {
+        stack = [screen]
+    }
+
+    func pop() {
+        guard stack.count > 1 else { return }
+        stack.removeLast()
+    }
+
+    func popToRoot() {
+        stack = [.list]
+    }
+}