Keep the screen awake during workouts; fix watch timers freezing

- iPhone: disable the idle timer while the exercise detail screen is open,
  so the display no longer sleeps mid-set.
- Watch: the work/rest timers counted on a run-loop Timer that watchOS
  throttles in the Always-On (wrist-down) state, so they froze. Anchor both
  to a wall-clock Date rendered with SwiftUI's self-updating timer text;
  rest haptics + auto-advance now derive from the end time so they catch up
  after a stall instead of stalling.

Workouts/Views/WorkoutLogs/ExerciseView.swift

@@ -10,6 +10,7 @@
 import SwiftUI
 import SwiftData
 import Charts
+import UIKit
 
 struct ExerciseView: View {
     @Environment(SyncEngine.self) private var sync
@@ -66,6 +67,11 @@ struct ExerciseView: View {
         .onAppear {
             refreshDocIfNeeded()
             progress = log?.currentStateIndex ?? 0
+            // Keep the screen lit while logging sets — a mid-workout sleep is annoying.
+            UIApplication.shared.isIdleTimerDisabled = true
+        }
+        .onDisappear {
+            UIApplication.shared.isIdleTimerDisabled = false
         }
         // Reflect external changes (e.g. a set completed on the watch) live. Each edit
         // rewrites the whole workout file, so the cache always holds the latest — pulling