fix(background): step-by-step prompt + Default preset (#217)

Bare BACKGROUND now opens an interactive colour prompt (via
ValuePromptCommand, same idiom as PDMODE/LTSCALE) instead of just
printing usage, so the command works both step-by-step and as a
one-shot `BACKGROUND <colour>`. Options reordered to
Default/Black/DarkGray/Gray/LightGray/White; DEFAULT restores the app
default, now unified to rgb(33,40,48) across startup, load and reset.
Also drop the unreachable OPTIONS arm in dispatch_display.

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

src/app/commands/display.rs

@@ -821,7 +821,7 @@ impl OpenCADStudio {
             }
 
             "POINTCLOUDATTACH" | "RECAP" | "SYNCPVIEWPORTS" | "UNDERLAYLAYERS" | "OBJECTSCALE"
-            | "UOSNAP" | "OPTIONS" => {
+            | "UOSNAP" => {
                 self.command_line
                     .push_info(&format!("{cmd}: not yet implemented."));
             }

src/app/commands/fileops.rs

@@ -228,17 +228,30 @@ impl OpenCADStudio {
 
             // ── Background color ───────────────────────────────────────────
             // Usage:  BACKGROUND <r> <g> <b>      (0–255 each)
-            //         BACKGROUND WHITE|BLACK|GRAY|DARKGRAY|LTGRAY   (preset)
-            //         BACKGROUND RESET            (restore default)
+            //         BACKGROUND DEFAULT|BLACK|DARKGRAY|GRAY|LIGHTGRAY|WHITE  (preset)
+            //         BACKGROUND DEFAULT          (restore the app default, rgb 33,40,48)
             // The chosen colour is also stored as the persisted default
             // (`default_bg_color` / `default_paper_bg_color`) so it survives
             // restarts and applies to new drawings (#188).
-            cmd if cmd == "BACKGROUND" || cmd.starts_with("BACKGROUND ") => {
+            // Bare BACKGROUND enters an interactive prompt for the colour, so
+            // the command works both as a one-shot (`BACKGROUND BLACK`) and as a
+            // type-then-choose flow (`BACKGROUND` ⏎, then `BLACK`). The prompt
+            // delegates back to the inline handler below via `Dispatch`.
+            "BACKGROUND" => {
+                use crate::command::ValuePromptCommand;
+                let c = ValuePromptCommand::new(
+                    "BACKGROUND",
+                    "BACKGROUND  colour [Default/Black/DarkGray/Gray/LightGray/White] or R G B (0–255):",
+                );
+                self.command_line.push_info(&c.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(c));
+            }
+            cmd if cmd.starts_with("BACKGROUND ") => {
                 let args = cmd.split_whitespace().skip(1).collect::<Vec<_>>();
                 let is_paper = self.tabs[i].scene.current_layout != "Model";
                 if args
                     .first()
-                    .map(|s| s.eq_ignore_ascii_case("RESET"))
+                    .map(|s| s.eq_ignore_ascii_case("DEFAULT") || s.eq_ignore_ascii_case("RESET"))
                     .unwrap_or(false)
                 {
                     if is_paper {
@@ -247,7 +260,7 @@ impl OpenCADStudio {
                         self.default_paper_bg_color = None;
                     } else {
                         self.tabs[i].bg_color = None;
-                        self.tabs[i].scene.bg_color = [0.11, 0.11, 0.11, 1.0];
+                        self.tabs[i].scene.bg_color = [33.0 / 255.0, 40.0 / 255.0, 48.0 / 255.0, 1.0];
                         self.default_bg_color = None;
                     }
                     // Wire colour adaptation (`adapt_to_bg`) reads the bg
@@ -285,7 +298,7 @@ impl OpenCADStudio {
                     // `persist_settings_if_changed()`.
                 } else {
                     self.command_line.push_info(
-                        "Usage: BACKGROUND <r> <g> <b> (0–255) | WHITE|BLACK|GRAY|DARKGRAY|LTGRAY | RESET",
+                        "Usage: BACKGROUND <r> <g> <b> (0–255) | DEFAULT|BLACK|DARKGRAY|GRAY|LIGHTGRAY|WHITE",
                     );
                 }
             }

src/scene/mod.rs

@@ -212,7 +212,7 @@ pub fn build_derived_caches(doc: &CadDocument) -> DerivedCaches {
     // bg is only relevant after the user enters a paper layout, and
     // `synced_hatch_models` re-runs `render_style` per-frame anyway so
     // the per-layout adaptation kicks in later regardless).
-    const LOAD_BG: [f32; 4] = [0.11, 0.11, 0.11, 1.0];
+    const LOAD_BG: [f32; 4] = [33.0 / 255.0, 40.0 / 255.0, 48.0 / 255.0, 1.0];
 
     // hatches
     let hatches: HashMap<Handle, HatchModel> = hatch_handles
@@ -946,7 +946,7 @@ impl Scene {
             solid_models: HashMap::default(),
             images: HashMap::default(),
             active_viewport: None,
-            bg_color: [0.11, 0.11, 0.11, 1.0],
+            bg_color: [33.0 / 255.0, 40.0 / 255.0, 48.0 / 255.0, 1.0],
             paper_bg_color: [1.0, 1.0, 1.0, 1.0],
             local_extent_max: 1e9,
             local_center: [0.0, 0.0],