Add edit feature enable/disable setting

- Added checkbox in Settings screen to enable/disable edit feature
- Setting stored in app_config.json as 'edit_feature_enabled'
- Edit workflow now validates: player setting, media type, server permission, card auth
- Shows appropriate error message when edit is blocked at any validation step
- Defaults to enabled (true) if not set
- All conditions must be met for edit interface to open

src/signage_player.kv

@@ -493,6 +493,36 @@
                 write_tab: False
                 on_touch_down: root.on_input_touch(self, args[1]) if self.collide_point(*args[1].pos) else None
         
+        # Edit Feature Enable/Disable
+        BoxLayout:
+            orientation: 'horizontal'
+            size_hint_y: None
+            height: dp(40)
+            spacing: dp(10)
+            
+            Label:
+                text: 'Enable Edit Feature:'
+                size_hint_x: 0.3
+                text_size: self.size
+                halign: 'left'
+                valign: 'middle'
+                
+            CheckBox:
+                id: edit_enabled_checkbox
+                size_hint_x: None
+                width: dp(40)
+                active: True
+                on_active: root.on_edit_feature_toggle(self.active)
+            
+            Label:
+                text: '(Allow editing images on this player)'
+                size_hint_x: 0.4
+                font_size: sp(12)
+                text_size: self.size
+                halign: 'left'
+                valign: 'middle'
+                color: 0.7, 0.7, 0.7, 1
+        
         Widget:
             size_hint_y: 0.05