feat(settings): editing during a session is the creator's call (default: allowed)

Creator ruling 2026-11-12: "we don't care if a creator wants to edit their scene: move stuff around,
shrink/grow a webcam view, etc. Why not?" Correct — the app was only ever *forcing* EDIT off at
go-live, never enforcing it, so the gun-safety was a habit rather than a rule.

App.LockEditWhileLive ships off: a show no longer takes EDIT away. Turning it on restores the old
behaviour exactly — go-live flips EDIT off and stop flips it back, and only because the lock took it,
so a creator who had already turned EDIT off does not get it handed back on stop.

MainViewModel.ApplyEditLockForSession holds the rule so it is testable without starting a broadcast;
EditWhileLiveTests covers both settings, the round trip, and the "don't hand it back" case. Settings →
Editing carries the checkbox.

This is the first slice of TASK 68 (editing while live).

678 pass, 0 warnings. Scope justification: Assets/starting-background.jpg is outside this unit — the
creator's replacement art, still awaiting their visual check.

ViewModels/MainViewModel.cs

@@ -347,6 +347,7 @@ public partial class MainViewModel : ViewModelBase
         _activeLayoutPath = _layoutStore.ActivePath;
         _toastPosition = _layoutStore.LoadNotificationPosition();
         _stickyAlertsUntilDismissed = _layoutStore.LoadStickyAlertsUntilDismissed();
+        _lockEditWhileLive = _layoutStore.LoadLockEditWhileLive();
         _recordFolder = _layoutStore.LoadRecordFolder();
 
         // What the last few sessions on this tier actually held, if it is worth asking about.