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.

Controls/SettingsDrawer.xaml

@@ -143,6 +143,13 @@
             </DockPanel>
 
             <!-- ═══ LAYOUT DEFAULTS (TASK 37) ═══ -->
+            <TextBlock Text="Editing" Style="{StaticResource SectionHeader}" Margin="0,10,0,4"/>
+            <CheckBox IsChecked="{Binding LockEditWhileLive, Mode=TwoWay}" Margin="10,0,0,2"
+                      Style="{StaticResource PropertyPillCheck}"
+                      Content="Lock editing while recording or live"/>
+            <TextBlock Text="Off: layers can be moved or resized during a show. On: going live turns EDIT off and stopping turns it back on."
+                       Style="{StaticResource PropertyHint}" Margin="28,0,0,10"/>
+
             <TextBlock Text="Layout defaults" Style="{StaticResource SectionHeader}" Margin="0,10,0,4"/>
             <TextBlock Text="Replaces every scene and layer with this build’s factory layout. Your selected webcam stays the same."
                        Style="{StaticResource PropertyHint}" Margin="10,0,0,6"/>