fix(sheetset): safer sheet edits

- Place on Sheet gives its viewport a unique id (shared
  `Scene::next_viewport_id`, also used by the normal viewport commit).
- Renaming a sheet's layout in an open tab records undo and marks the tab
  modified; in a closed drawing the previous file is kept as `.bak`
  (ISAVEBAK) before it is written back.
- New Sheet saves the drawing before the `.dst`: a failed save leaves no
  sheet behind.
- Rename & Renumber Previous/Next no longer panics on an empty list.
- Leftovers: unused binding, a hard-coded localized Documents path.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/scene/mod.rs

@@ -4458,6 +4458,22 @@ impl Scene {
         self.current_layout_block_handle()
     }
 
+    /// A unique id for a new viewport on the current layout: one past the
+    /// largest id there, at least 2 (1 is the layout's own paper viewport).
+    pub fn next_viewport_id(&self) -> i16 {
+        let layout_block = self.current_layout_block_handle();
+        let max_id = self
+            .document
+            .entities()
+            .filter_map(|e| match e {
+                codec::EntityType::Viewport(v) if v.common.owner_handle == layout_block => Some(v.id),
+                _ => None,
+            })
+            .max()
+            .unwrap_or(1);
+        (max_id + 1).max(2)
+    }
+
     /// True when an entity belongs to the active layout, including imported
     /// DXF entities whose owner handle is NULL but whose BlockRecord still
     /// lists the entity. Keep command validation aligned with the same