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.Streaming.Operations.cs

@@ -298,7 +298,7 @@ public partial class MainViewModel : ViewModelBase
         // before that (2026-09-25 diagnosis), so chat resolution runs AFTER the pump
         // starts. Chat is non-fatal — if it fails, the stream continues without chat.
         LiveScene = StagedScene;
-        EditingActive = false;
+        ApplyEditLockForSession(starting: true);
         await _framePump.StartAsync(); // never throws; failures log + surface via Failed
 
         // Check promptly until YouTube confirms the broadcast is live. Health
@@ -424,7 +424,7 @@ public partial class MainViewModel : ViewModelBase
         _sessionStartedAtUtc = null;
         IsRecording = false;
         LiveScene = null;
-        EditingActive = true;
+        ApplyEditLockForSession(starting: false);
         // TASK 41: a stopped test broadcast forgets its drawer session (idempotent)
         // and drops the TEST face — a test never outlives its run.
         TestSession.EndTestSilently();