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 <noreply@anthropic.com>

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."));
             }