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/Task.swift

@@ -0,0 +1,54 @@
+import Foundation
+
+/// A planned task with a title, optional link, and an append-only history of step lists.
+/// Named `TaskPlan` to avoid collision with `_Concurrency.Task`.
+struct TaskPlan: Codable, Identifiable, Hashable, Sendable {
+    let id: String
+    var title: String
+    var date: Date
+    var link: String?
+    var stepHistory: [[Step]]
+
+    init(id: String, title: String, date: Date = Date(), link: String? = nil, stepHistory: [[Step]] = []) {
+        self.id = id
+        self.title = title
+        self.date = date
+        self.link = link
+        self.stepHistory = stepHistory
+    }
+
+    var steps: [Step] {
+        stepHistory.last ?? []
+    }
+
+    var initialPlan: [Step] {
+        stepHistory.first ?? []
+    }
+
+    var wasUpdated: Bool {
+        stepHistory.count > 1
+    }
+
+    var totalEstimation: Int {
+        steps.reduce(0) { $0 + $1.estimation }
+    }
+
+    mutating func newSteps(_ steps: [Step]) {
+        stepHistory.append(steps)
+    }
+
+    mutating func editSteps(_ steps: [Step]) {
+        stepHistory.append(self.steps + steps)
+    }
+
+    mutating func updateSteps(_ steps: [Step]) {
+        stepHistory.append(steps)
+    }
+
+    mutating func removeStep(at index: Int) {
+        guard index >= 0, index < steps.count else { return }
+        stepHistory.append(steps.enumerated().compactMap { i, step in
+            i == index ? nil : step
+        })
+    }
+}