feat: restore and persist saved camera position per layout

- On file open and layout tab switch, restore the camera from the
  document's saved view (*Active VPort for model space, sheet viewport
  for paper space) instead of fitting to wire bounds.
- After any zoom/pan/rotate, write the new camera position back to the
  document and mark the file dirty so it is saved with the file.
- Uses last_synced_camera_gen to avoid marking the file dirty on the
  initial camera restore.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/document.rs

@@ -42,6 +42,8 @@ pub(super) struct DocumentTab {
     pub(super) refedit_session: Option<RefEditSession>,
     /// Currently active MLeader style name.
     pub(super) active_mleader_style: String,
+    /// Last camera_generation value written back to the document.
+    pub(super) last_synced_camera_gen: u64,
 }
 
 impl DocumentTab {
@@ -83,6 +85,7 @@ impl DocumentTab {
             paper_bg_color: None,
             refedit_session: None,
             active_mleader_style: "Standard".to_string(),
+            last_synced_camera_gen: 0,
         }
     }