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/mtext_editor.rs

@@ -111,6 +111,9 @@ pub struct MTextEditorState {
     pub rect_width: f64,
     /// `Some` when editing an existing entity; `None` for a fresh MText.
     pub editing: Option<Handle>,
+    /// When true the panel shows the rendered preview; when false the raw
+    /// code/text input. Toggled so the two never stack.
+    pub show_preview: bool,
     /// Canvas-space anchor where the toolbar + text area are drawn (the
     /// insertion-point click position).
     pub screen_anchor: iced::Point,
@@ -132,6 +135,7 @@ impl MTextEditorState {
             width: "1".to_string(),
             char_space: "0".to_string(),
             preview_wires: Vec::new(),
+            show_preview: false,
             attachment: AttachmentPoint::TopLeft,
             line_spacing: 1.0,
             // Default box ~20 characters wide; overwritten with the entity's