Add an "End Workout" flow for partially-done workouts

Replace the in-workout "+" toolbar button with an ellipsis menu offering
"Add Exercise" and "End Workout". Ending opens a Save/Discard action sheet:
Save marks the remaining exercises as skipped and resolves the workout to
completed (stamping end), which drops it off the watch's active list and ends
the watch's HealthKit session; Discard soft-deletes it.

Teach the status-from-logs derivation that a skipped log is terminal, and
consolidate the three duplicated copies into a single shared
WorkoutDocument.recomputeStatusFromLogs() so an ended workout stays finished
regardless of which screen the next edit comes from.

Claude-Session: https://claude.ai/code/session_01SCv7zvGFcKy47KSTnTLxRe

Shared/Model/Documents.swift

@@ -69,6 +69,33 @@ struct WorkoutDocument: Codable, Sendable, Equatable, Identifiable {
     }
 }
 
+extension WorkoutDocument {
+    /// Derive the aggregate `status` + `end` from the current logs. A log that is
+    /// `.completed` or `.skipped` counts as *resolved*; a workout whose logs are all
+    /// resolved is finished (`.completed`, with `end` stamped). This is the single
+    /// source of the status-from-logs rule — every screen that mutates logs calls it,
+    /// so an ended workout (remaining exercises skipped) stays finished no matter which
+    /// screen the next edit comes from.
+    mutating func recomputeStatusFromLogs() {
+        let statuses: [WorkoutStatus] = logs.map { WorkoutStatus(rawValue: $0.status) ?? .notStarted }
+        let isResolved: (WorkoutStatus) -> Bool = { $0 == .completed || $0 == .skipped }
+
+        let allResolved = !statuses.isEmpty && statuses.allSatisfy(isResolved)
+        let anyStarted = statuses.contains { $0 != .notStarted }
+
+        if allResolved {
+            status = WorkoutStatus.completed.rawValue
+            end = Date()
+        } else if anyStarted {
+            status = WorkoutStatus.inProgress.rawValue
+            end = nil
+        } else {
+            status = WorkoutStatus.notStarted.rawValue
+            end = nil
+        }
+    }
+}
+
 struct WorkoutLogDocument: Codable, Sendable, Equatable, Identifiable {
     var id: String                 // ULID
     var exerciseName: String