feat: per-viewport background color (BACKGROUND command)

- DocumentTab: bg_color / paper_bg_color Optional<[f32;4]> fields
- view.rs: use custom color when set, fall back to defaults
- BACKGROUND <r> <g> <b>: set model or paper-space background (0–255 RGB)
- BACKGROUND RESET: restore default dark grey / paper grey

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/document.rs

@@ -31,6 +31,10 @@ pub(super) struct DocumentTab {
     pub(super) active_layer: String,
     /// Currently active UCS. `None` means WCS (identity transform).
     pub(super) active_ucs: Option<Ucs>,
+    /// Custom model-space background color.  `None` = default dark grey.
+    pub(super) bg_color: Option<[f32; 4]>,
+    /// Custom paper-space background color.  `None` = default off-white grey.
+    pub(super) paper_bg_color: Option<[f32; 4]>,
 }
 
 impl DocumentTab {
@@ -56,6 +60,8 @@ impl DocumentTab {
             history: HistoryState::default(),
             active_layer: "0".to_string(),
             active_ucs: None,
+            bg_color: None,
+            paper_bg_color: None,
         }
     }