feat(field): FIELDDISPLAY gray field background (screen only, profile setting)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/settings.rs

@@ -371,6 +371,8 @@ pub struct UserSettings {
     pub image_frame: i16,
     /// The Attribute Definition dialog's "Specify on-screen" choice.
     pub attdef_on_screen: bool,
+    /// FIELDDISPLAY: fields show on a gray background.
+    pub field_display: bool,
     /// Nested-copy symbol handling: false inserts, true binds.
     pub ncopy_bind: bool,
     /// Last Options page; unknown saved names fall back without rejecting the config.
@@ -750,6 +752,7 @@ impl Default for UserSettings {
             grip_object_limit: DEFAULT_GRIP_OBJECT_LIMIT,
             image_frame: 1,
             attdef_on_screen: true,
+            field_display: true,
             ncopy_bind: false,
             cursor_type: CursorType::Crosshair,
             crosshair_color: None,