feat(commands): prompt for a system-variable value on the next line

Typing a bare system-variable name (MIRRTEXT, FILLMODE, LTSCALE, …) now
echoes the current value and prompts "Enter new value for <NAME> <cur>:",
capturing the next command-line entry as the new value — Enter keeps the
current one, Escape cancels. The single-line "MIRRTEXT 1" form still works.
Mirrors the awaiting_vports pattern: a pending_setvar field on the app,
consumed at the top of on_command_submit.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/mod.rs

@@ -274,6 +274,10 @@ pub(super) struct OpenCADStudio {
     /// `true` after a bare `VPORTS` in model space — the next command-line
     /// entry is treated as the tiled-config option (SIngle/2H/2V/4).
     awaiting_vports: bool,
+    /// Set to the variable name after a bare system-variable query (e.g. a lone
+    /// `MIRRTEXT`) — the next command-line entry is its new value, empty keeps
+    /// the current one.
+    pending_setvar: Option<String>,
     /// Cursor is hovering over the UCS icon body — drives the hover highlight.
     ucs_icon_hover: bool,
     /// UCS icon is selected (clicked): its grips are shown and draggable.
@@ -1958,6 +1962,7 @@ impl OpenCADStudio {
             default_bg_color: None,
             default_paper_bg_color: None,
             awaiting_vports: false,
+            pending_setvar: None,
             ucs_icon_hover: false,
             ucs_icon_selected: false,
             ucs_grip_drag: None,