Give saving its own page, with the two settings that had no control

General had become three unrelated controls: a language picker, a save format
and a file association. The last two move to an Open and Save page, where
they join the two saving preferences that have never had a widget.

Automatic save has worked since the SAVETIME command was added and is
persisted; the only way to change the interval was to type it. Same for the
`.bak` copy from #205 and its ISAVEBAK command. Both are settings people
expect to find in a dialog, and neither was in one.

The page also carries a button through to the Plot dialog. Options holds the
preferences; the plot device, paper and plot styles stay where they already
work well. Note that the modal stack shows one dialog at a time, so the
button replaces Options rather than stacking on it.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/app/update/mod.rs

@@ -6188,6 +6188,18 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::SaveTimeChanged(minutes) => {
+                self.savetime_min = minutes.max(0);
+                self.persist_settings_if_changed();
+                Task::none()
+            }
+
+            Message::BackupOnSaveChanged(enabled) => {
+                self.backup_on_save = enabled;
+                self.persist_settings_if_changed();
+                Task::none()
+            }
+
             Message::PickDragRectToggled(rectangle) => {
                 self.pick_drag_rect = rectangle;
                 self.persist_settings_if_changed();