feat(mtext): edit text directly in the preview with a caret

The rendered MText preview is now an editable surface: it shows a text caret,
click to position it between glyphs, and type/Backspace/Delete/Enter to edit.
Left/Right arrows move the caret; drag-selection + toolbar formatting still
work and typing replaces a selection.

Keystrokes are routed by mapping visible-character offsets to byte spans in
the value (raw newlines count as paragraph breaks, matching the layout). While
the preview is open the command line releases its on_input so typed glyphs
reach the preview instead, and the caret starts at the end of the text so
typing works without a click first.

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

src/app/mod.rs

@@ -744,6 +744,14 @@ 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.
     MTextOk,
     /// Discard the editor without creating / changing the entity.