feat: Implement in-place MText editor with formatting toolbar and live preview

MTEXT (and double-clicking an MText) now opens an in-place editor anchored
at the insertion point instead of typing a single line on the command line.

The editor has a two-row formatting toolbar: text style / font / height,
bold, italic, underline, overline, strike, upper/lowercase and a colour
dropdown on top; oblique angle, width factor, character spacing, paragraph
alignment, justification, line spacing and OK/Cancel below. Character
toggles wrap the selection in DXF inline codes; global fields apply as
leading runs or entity properties.

Editing is shown in the editor's own area, not on the drawing: a Code/Preview
toggle switches between the raw text input and a rendered preview canvas that
draws the tessellated glyph strokes at real size, scrolling when the text
outgrows the view. The MText box keeps a fixed width so text wraps to the
next line instead of stretching into one line.

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

src/app/mod.rs

@@ -737,6 +737,8 @@ pub enum Message {
     MTextAlign(mtext_editor::ParaAlign),
     /// Toolbar line-spacing change.
     MTextLineSpacing(f32),
+    /// Toggle the editor between the raw code input and the rendered preview.
+    MTextTogglePreview,
     /// Commit the editor: create or update the MText entity.
     MTextOk,
     /// Discard the editor without creating / changing the entity.