feat(live): Live Activity / Dynamic Island (WidgetKit extension iOS 16.2+/17+)

Nouvelle target "CoachLiveActivity" (Widget Extension, bundle
ch.hypnotruck.coach.LiveActivity, iOS 17.0) embarquée dans App via copy phase
"Embed App Extensions" (dstSubfolderSpec=13). API ActivityKit + WidgetKit
vérifiées sur SDK iOS 26.5 swiftinterface (Activity.request(content:),
ActivityConfiguration, DynamicIsland(expanded:compactLeading:trailing:minimal:)).

Architecture :
- CoachLiveActivityAttributes (modèle partagé App ↔ extension)
- CoachLiveActivityWidget (Lock Screen + Dynamic Island UI)
- CoachLiveActivityBundle (@main WidgetBundle)
- LiveActivityManager (App side, wrap Activity.request/update/end)
- LiveStore hook : start sur 1er sample, update à chaque ingest, end après
  60s "lost". Tout guardé @available(iOS 16.2, *) pour préserver deployment iOS 15.
- Info.plist App : NSSupportsLiveActivities = YES.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ios/App/CoachLiveActivity/CoachLiveActivityBundle.swift

@@ -0,0 +1,13 @@
+// CoachLiveActivityBundle.swift
+// Entry point @main de l'extension WidgetKit. Contient le ou les Widgets
+// exposés par cette extension. Ici juste la Live Activity du live workout.
+
+import WidgetKit
+import SwiftUI
+
+@main
+struct CoachLiveActivityBundle: WidgetBundle {
+    var body: some Widget {
+        CoachLiveActivityWidget()
+    }
+}