feat(annotative): annotation-scale manager window

Replace the scale popup's inline delete/add affordances with a single
"Manage scales..." row that opens a proper manager, matching the style
managers: it reuses the shared style-manager palette and chrome (toolbar +
list + editor frame), so it looks and behaves like the Text / Dim / MLeader
managers.

The manager lists the drawing's annotation scales (name + ratio, the current
one checked) and edits them through a Name / Paper units / Drawing units
editor: New adds a scale from the fields, Apply renames / re-ratios the
selected one, Delete removes it (never the current scale), and Set Current
makes the selected scale the model-space annotation scale. Backed by
Scene::add_scale / remove_scale / edit_scale on ACAD_SCALELIST.

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

src/app/view/mod.rs

@@ -1573,6 +1573,7 @@ impl OpenCADStudio {
             AttributeEditor => (640, 500),
             LayerDeleteWarning => (440, 200),
             Aliases => (480, 520),
+            ScaleManager => (520, 360),
         };
         Some((w as f32 + EXTRA_W, h as f32 + EXTRA_H))
     }