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/mod.rs

@@ -2527,6 +2527,19 @@ impl OpenCADStudio {
 
             Message::UnsavedDialogSave => self.on_unsaved_dialog_save(),
 
+            Message::AecDropSameVersion => self.on_aec_drop_same_version(),
+            Message::AecDropProceed => self.on_aec_drop_proceed(),
+            Message::AecDropBack => {
+                self.active_modal = Some(crate::app::ModalKind::SaveDialog);
+                Task::none()
+            }
+
+            Message::OverwriteConfirm => self.on_overwrite_confirm(),
+            Message::OverwriteCancel => {
+                self.active_modal = Some(crate::app::ModalKind::SaveDialog);
+                Task::none()
+            }
+
             Message::UnsavedPickedSavePath(Some(path)) => {
                 self.on_unsaved_picked_save_path_some(path)
             }