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/view/mod.rs

@@ -1358,6 +1358,8 @@ impl OpenCADStudio {
             DimStyle => (720, 560),
             AssocPrompt => (440, 210),
             Unsaved => (420, 160),
+            AecDropWarning => (480, 230),
+            OverwriteWarning => (420, 180),
             SaveDialog => (560, 480),
             PointStyle => (360, 470),
         };