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/commands/styleprops.rs

@@ -1501,6 +1501,27 @@ impl OpenCADStudio {
                     _ => self.command_line.push_error("Requires 0 or 1"),
                 }
             }
+            "SAVETIME" => {
+                self.command_line
+                    .push_output(&format!("SAVETIME = {}", self.savetime_min));
+            }
+            cmd if cmd.starts_with("SAVETIME ") => {
+                match cmd.trim_start_matches("SAVETIME").trim().parse::<i32>() {
+                    Ok(v) if v >= 0 => {
+                        self.savetime_min = v;
+                        self.persist_settings_if_changed();
+                        let msg = if v == 0 {
+                            "SAVETIME set to 0 (autosave off)".to_string()
+                        } else {
+                            format!("SAVETIME set to {v} minute(s)")
+                        };
+                        self.command_line.push_output(&msg);
+                    }
+                    _ => self
+                        .command_line
+                        .push_error("Requires a non-negative number of minutes (0 = off)"),
+                }
+            }
             "PDSIZE" => {
                 use crate::command::ValuePromptCommand;
                 let c = ValuePromptCommand::new(