feat: MLEADERSTYLE command — create and manage MultiLeader styles

Stores MultiLeaderStyle objects in document.objects. Supports:
  MLEADERSTYLE LIST — list all styles
  MLEADERSTYLE NEW <name> — create a new style
  MLEADERSTYLE SET <name> <prop> <val> — edit text_height,
    arrowhead_size, landing_distance, landing_gap
  MLEADERSTYLE CURRENT [<name>] — get/set active style

Active style name tracked per-tab in active_mleader_style.

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

src/app/document.rs

@@ -40,6 +40,8 @@ pub(super) struct DocumentTab {
     pub(super) paper_bg_color: Option<[f32; 4]>,
     /// Active REFEDIT session, if any.
     pub(super) refedit_session: Option<RefEditSession>,
+    /// Currently active MLeader style name.
+    pub(super) active_mleader_style: String,
 }
 
 impl DocumentTab {
@@ -69,6 +71,7 @@ impl DocumentTab {
             bg_color: None,
             paper_bg_color: None,
             refedit_session: None,
+            active_mleader_style: "Standard".to_string(),
         }
     }