feat(io): autosave to .sv$ recovery files (SAVETIME command) (#205)

A periodic subscription (every SAVETIME minutes — default 10, 0 disables)
writes a <name>.sv$ recovery copy of each dirty, already-saved drawing at its
own DWG version. Non-destructive: it never touches the original file or the
dirty flag, and skips unsaved (path-less) tabs. The SAVETIME command
sets/queries the interval and persists it. Recovery files (.sv$ and .bak) open
through the normal Open dialog — the loader detects DWG vs DXF from the leading
magic bytes, and a "Backup / Autosave" filter lists them.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -2540,6 +2540,7 @@ impl OpenCADStudio {
                 self.active_modal = Some(crate::app::ModalKind::SaveDialog);
                 Task::none()
             }
+            Message::AutoSave => self.on_autosave(),
 
             Message::UnsavedPickedSavePath(Some(path)) => {
                 self.on_unsaved_picked_save_path_some(path)