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/mod.rs

@@ -332,6 +332,13 @@ pub(super) struct OpenCADStudio {
     /// The open in-canvas modal dialog, if any (Plan B: shared overlay instead
     /// of OS windows).
     active_modal: Option<ModalKind>,
+    /// Set once the user acknowledges the AEC-drop warning, so re-entering the
+    /// save path proceeds instead of re-showing the warning.
+    aec_drop_acknowledged: bool,
+    /// Number of unsupported objects shown in the AEC-drop warning modal.
+    aec_drop_count: usize,
+    /// Set once the user confirms overwriting an existing file on Save-As.
+    overwrite_acknowledged: bool,
     /// Pixel offset of the active modal from screen-centre (drag-to-move).
     /// Reset to zero whenever a modal closes so each dialog opens centred.
     modal_offset: iced::Vector,
@@ -923,6 +930,8 @@ pub enum ModalKind {
     DimStyle,
     Unsaved,
     SaveDialog,
+    AecDropWarning,
+    OverwriteWarning,
     #[cfg_attr(target_arch = "wasm32", allow(dead_code))]
     AssocPrompt,
     PointStyle,
@@ -1093,6 +1102,18 @@ pub enum Message {
     UnsavedDialogDiscard,
     /// User clicked "Cancel" in the unsaved-changes dialog.
     UnsavedDialogCancel,
+    // ── AEC / unsupported-object drop warning (lossy Save-As) ──────────────
+    /// Save as the source DWG version so the unsupported objects survive.
+    AecDropSameVersion,
+    /// Proceed with the chosen target format, dropping the unsupported objects.
+    AecDropProceed,
+    /// Go back to the Save dialog from the AEC-drop warning.
+    AecDropBack,
+    // ── Overwrite confirmation (Save-As over an existing file) ────────────
+    /// Replace the existing file and save.
+    OverwriteConfirm,
+    /// Go back to the Save dialog from the overwrite warning.
+    OverwriteCancel,
     /// Save-as path picked for the unsaved-changes → save → close flow.
     UnsavedPickedSavePath(Option<std::path::PathBuf>),
     // ─────────────────────────────────────────────────────────────────────
@@ -1868,6 +1889,9 @@ impl OpenCADStudio {
             main_window: None,
             color_pick_target: None,
             active_modal: None,
+            aec_drop_acknowledged: false,
+            aec_drop_count: 0,
+            overwrite_acknowledged: false,
             modal_offset: iced::Vector::ZERO,
             modal_drag_last: None,
             modal_dragging: false,