feat(text): MTEXT background fill / mask + text frame, editable in Properties

Render the MTEXT background (never drawn before — the flags were only shown
read-only in the panel) and make it editable.

- Text arm emits an opaque fill_tris rect behind the glyphs: flag 0x01 uses
  the entity background colour, 0x02 uses the canvas colour (a wipeout-style
  mask). Box = glyph bounds padded by (background_scale - 1) × text height.
- Text frame (flag 0x10) draws a closed rectangle in the text colour around
  the same box.
- Threaded `bg_color` into `tessellate()` (+ all callers and the
  MultiLeaderTess trait) so the mask can use the drawing-window colour.
- Properties: "Background mask" is a Off / Fill / Mask dropdown, "Background
  color" is its own colour picker (independent of the entity colour — new
  PropBgColor* messages; picking a colour switches to Fill mode), and "Text
  frame" is a checkbox toggle. Edits re-tessellate so the change shows live.

Renders via the face3d fill path, so the fill shows with FILLMODE on (same as
MLEADER background fill); the frame is a stroke and always shows.

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

src/app/mtext_editor.rs

@@ -396,6 +396,7 @@ impl super::OpenCADStudio {
         let mt = ed.build_mtext();
         let entity = EntityType::MText(mt.clone());
         let anno = self.tabs[i].scene.annotation_scale;
+        let bg = self.tabs[i].scene.bg_color;
         let wires: Vec<WireModel> = tessellate::tessellate(
             &self.tabs[i].scene.document,
             ed.editing.unwrap_or(Handle::new(1)),
@@ -407,6 +408,7 @@ impl super::OpenCADStudio {
             1.0,
             anno,
             None,
+            bg,
         );
         // Per-character boxes share the preview wires' absolute coordinate frame.
         let boxes = crate::entities::mtext::glyph_boxes(&mt, &self.tabs[i].scene.document);