feat(save): warn before dropping AEC/Civil objects or overwriting on Save-As

Two in-canvas confirmations on the Save-As path:

- AEC/Civil drop warning: when the drawing carries unsupported AEC/Civil
  (application) objects that survive only as verbatim source-version bytes,
  saving to a different version or to DXF would silently drop them. The
  dialog reports the count and offers "Save in source version" (keep) vs
  "Save anyway" (drop). Same-version DWG saves are lossless and skip it.
- Overwrite confirmation: when the chosen filename already exists in the
  target folder, "Replace" / "Cancel" before clobbering it.

The two chain (AEC first, then overwrite) via per-flow acknowledgement flags
reset whenever the Save dialog opens or closes. Detection is a new
io::dropped_on_save_count over the document's Unknown (raw-passthrough)
objects/entities.

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

src/app/update/dialog.rs

@@ -49,12 +49,16 @@ impl OpenCADStudio {
             self.save_dialog_filename = format!("{}.{ext}", self.tabs[tab_idx].tab_display_name());
         }
         self.save_dialog_entries = crate::io::read_dir_entries(&self.save_dialog_folder.clone());
+        self.aec_drop_acknowledged = false;
+        self.overwrite_acknowledged = false;
         self.active_modal = Some(crate::app::ModalKind::SaveDialog);
         Task::none()
     }
 
 
     pub(in crate::app) fn close_save_dialog_window(&mut self) -> Task<Message> {
+        self.aec_drop_acknowledged = false;
+        self.overwrite_acknowledged = false;
         if self.active_modal == Some(crate::app::ModalKind::SaveDialog) {
             self.active_modal = None;
         }