feat: add style selector comboboxes to Annotate ribbon tab

New StyleComboGroup RibbonItem renders a dropdown combobox for selecting
text style, dimension style, multileader style, or table style. Ribbon
groups in the Annotate tab now match the reference layout: each group
has a large tool on the left and a style combobox with tool rows on the
right.

Each combobox shows the active style name, opens a scrollable item list
on click, and has an optional "Manage…" row that fires the style manager
command (STYLE / DIMSTYLE / MLEADERSTYLE / TABLESTYLE).

Style state is synced from the document on every layer refresh via
sync_ribbon_styles(). RibbonStyleChanged message propagates selections
back to the acadrust document header and the active_mleader_style field.

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

src/app/mod.rs

@@ -295,6 +295,8 @@ pub enum Message {
     RibbonLinetypeChanged(String),
     /// User changed the active lineweight in the Properties toolbar.
     RibbonLineweightChanged(LineWeight),
+    /// User selected a style from a style combobox in the ribbon.
+    RibbonStyleChanged { key: crate::modules::StyleKey, name: String },
 
     // ── Properties panel ──────────────────────────────────────────────────
     /// User selected a layer from the layer pick_list in the Properties panel.