chore(mtext): drop unused preview-edit message variants

MTextType/MTextBackspace/MTextDelete were never emitted — preview typing,
backspace and delete route through the existing CommandAppendChar /
CommandBackspace / DeleteSelected handlers, which call the methods directly.
Removes the dead variants and their arms (fixes a never-constructed warning).

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

src/app/mod.rs

@@ -744,12 +744,6 @@ pub enum Message {
     MTextSelStart(usize),
     /// Extend the preview selection to the given visible-character offset.
     MTextSelTo(usize),
-    /// Type text at the preview caret (replacing any selection).
-    MTextType(String),
-    /// Backspace at the preview caret.
-    MTextBackspace,
-    /// Delete-forward at the preview caret.
-    MTextDelete,
     /// Move the preview caret by N visible characters.
     MTextCaretMove(i32),
     /// Commit the editor: create or update the MText entity.