fix(ribbon): load style/layer dropdowns from the document

The Annotate style dropdowns (text / dimension / multileader / table) and
the layer dropdown were hard-coded to "Standard" / "0" and never synced
from the opened document, so they showed placeholder values even with no
file open and stale values after closing one.

- Start the ribbon's style and layer lists empty instead of with a
  placeholder entry.
- Sync the style dropdowns from the document on file open and on tab
  new/switch/close (the layer sync already ran there).
- Guard sync_ribbon_layers / sync_ribbon_styles / sync_ribbon_from_selection
  for the Start (welcome) tab so all three dropdowns stay empty there.
- When a style family is absent (e.g. no table/multileader styles), fall
  back to empty rather than a non-existent "Standard".

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

src/ui/ribbon/mod.rs

@@ -74,15 +74,10 @@ impl Ribbon {
             ortho_mode: true,
             open_dropdown: None,
             last_cmd: HashMap::default(),
-            layer_names: vec!["0".to_string()],
-            active_layer: "0".to_string(),
-            layer_infos: vec![LayerInfo {
-                name: "0".to_string(),
-                color: Color::WHITE,
-                visible: true,
-                frozen: false,
-                locked: false,
-            }],
+            // Empty until a document is open — populated by sync_ribbon_layers.
+            layer_names: vec![],
+            active_layer: String::new(),
+            layer_infos: vec![],
             active_color: AcadColor::ByLayer,
             active_linetype: "ByLayer".to_string(),
             active_lineweight: LineWeight::ByLayer,
@@ -91,14 +86,16 @@ impl Ribbon {
                 art: String::new(),
             }],
             prop_color_palette_open: false,
-            text_style_names: vec!["Standard".to_string()],
-            active_text_style: "Standard".to_string(),
-            dim_style_names: vec!["Standard".to_string()],
-            active_dim_style: "Standard".to_string(),
-            mleader_style_names: vec!["Standard".to_string()],
-            active_mleader_style: "Standard".to_string(),
-            table_style_names: vec!["Standard".to_string()],
-            active_table_style: "Standard".to_string(),
+            // Empty until a document is open — the Annotate style dropdowns
+            // are populated from the active document by sync_ribbon_styles.
+            text_style_names: vec![],
+            active_text_style: String::new(),
+            dim_style_names: vec![],
+            active_dim_style: String::new(),
+            mleader_style_names: vec![],
+            active_mleader_style: String::new(),
+            table_style_names: vec![],
+            active_table_style: String::new(),
         }
     }