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 <noreply@anthropic.com>

src/app/properties.rs

@@ -3417,25 +3417,7 @@ handles={handles_ms:.1} panel={:.1} ribbon={ribbon_ms:.1} tail={:.1} selected={}
         {
             // Assign a unique viewport ID (max existing id + 1, min 2).
             if let codec::EntityType::Viewport(ref mut vp) = entity {
-                let layout_block = self.tabs[i].scene.current_layout_block_handle_pub();
-                let max_id = self.tabs[i]
-                    .scene
-                    .document
-                    .entities()
-                    .filter_map(|e| {
-                        if let codec::EntityType::Viewport(v) = e {
-                            if v.common.owner_handle == layout_block {
-                                Some(v.id)
-                            } else {
-                                None
-                            }
-                        } else {
-                            None
-                        }
-                    })
-                    .max()
-                    .unwrap_or(1);
-                vp.id = (max_id + 1).max(2);
+                vp.id = self.tabs[i].scene.next_viewport_id();
             }
 
             let layout = self.tabs[i].scene.current_layout.clone();