Frames: state-following ribbon list, DWF/DGN frames, image frame encoding and profile, frame-only picking

- The Frames list shows the drawing's frame state (hide / display and plot /
  display but not plot / frames vary), follows commands, undo and document
  switches, and lists the mixed state as a disabled item.
- DWFFRAME and DGNFRAME are frame variables of their own, kept in the header,
  and FRAME sets them with the others.
- Out-of-range frame values say "Requires an integer between 0 and 2." and
  ask again; a set value reports "Regenerating model.".
- IMAGEFRAME is stored in the image variables as bits (2 is written as 3) and
  a drawing without them uses the profile value, which setting IMAGEFRAME
  updates and the settings file keeps.
- Raster images, PDF underlays and wipeouts are picked on their frame only;
  underlay snap geometry is never picked.

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

src/app/settings.rs

@@ -366,6 +366,9 @@ pub struct UserSettings {
     /// GRIPOBJLIMIT: past this many selected objects, no grips are drawn at
     /// all. 0 means no limit. The drawing header carries no slot for it.
     pub grip_object_limit: i32,
+    /// IMAGEFRAME of the profile: used by new drawings and by drawings that
+    /// carry no image variables of their own.
+    pub image_frame: i16,
     /// Nested-copy symbol handling: false inserts, true binds.
     pub ncopy_bind: bool,
     /// Last Options page; unknown saved names fall back without rejecting the config.
@@ -653,6 +656,7 @@ impl Default for UserSettings {
             right_click_mode: RightClickMode::ShortcutMenu,
             right_click_hold_ms: 250,
             grip_object_limit: DEFAULT_GRIP_OBJECT_LIMIT,
+            image_frame: 1,
             ncopy_bind: false,
             cursor_type: CursorType::Crosshair,
             crosshair_color: None,