fix(commandline): pin active step prompt instead of fading

The current step's prompt was written into the command-line widget
label. Instead, keep it as a normal history line but freeze its
cooldown: the step prompt is pinned so it does not fade while the user
is still on that step. When the step changes the previous line's
cooldown restarts; when the command ends the pin clears and it fades.

`update` is now a thin wrapper that mirrors the active command's prompt
into the command line after each message; the body moved to
`update_inner`.

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

src/app/view.rs

@@ -1177,13 +1177,8 @@ impl OpenCADStudio {
             && !tab.dyn_fields.is_empty())
             || self.mtext_editor.as_ref().is_some_and(|e| e.show_preview)
             || self.text_inline.is_some();
-        let active_prompt = tab.active_cmd.as_ref().map(|c| c.prompt());
         let command_line_overlay =
-            iced::widget::container(self.command_line.view(
-                allow_autocomplete,
-                dyn_capturing,
-                active_prompt,
-            ))
+            iced::widget::container(self.command_line.view(allow_autocomplete, dyn_capturing))
             .width(Fill)
             .height(Fill)
             .align_x(iced::alignment::Horizontal::Center)