feat(background): persist viewport background across sessions (#188)

The BACKGROUND command set a per-tab colour that was lost on restart and
didn't apply to new drawings, so there was no way to make a non-black
background stick. Persist the chosen model/paper background in UserSettings
(settings.txt) and re-apply it on launch and to every drawing tab created
afterward.

- settings.rs: persist `bg_color` / `paper_bg_color` as 0–255 RGB triplets.
- App gains `default_bg_color` / `default_paper_bg_color`; current_settings /
  apply_settings round-trip them, and apply_settings pushes the restored
  colour onto existing tabs. New/opened/replaced tabs call `apply_bg_default`.
- BACKGROUND updates the persisted default, saved centrally via
  persist_settings_if_changed; RESET clears it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -245,6 +245,12 @@ pub(super) struct OpenCADStudio {
     dyn_input: bool,
     /// Controls whether the TEXTEDIT command repeats automatically (0 = Multiple, 1 = Single).
     pub texteditmode: bool,
+    /// Persisted default viewport background, restored from settings and applied
+    /// to every drawing tab (new and opened) so a chosen background survives
+    /// restarts (#188). `None` = the built-in dark-grey / off-white defaults.
+    /// The `a` channel is always 1.0.
+    default_bg_color: Option<[f32; 4]>,
+    default_paper_bg_color: Option<[f32; 4]>,
     /// `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,
@@ -1770,6 +1776,8 @@ impl OpenCADStudio {
             show_grid: false,
             dyn_input: true,
             texteditmode: false,
+            default_bg_color: None,
+            default_paper_bg_color: None,
             awaiting_vports: false,
             tile_drag: None,
             dyn_user_reshaped: false,