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 <noreply@anthropic.com>

src/app/view/modal.rs

@@ -192,6 +192,10 @@ impl OpenCADStudio {
                             pick_drag_rect: self.pick_drag_rect,
                             grip_object_limit: self.grip_object_limit,
                         },
+                        crate::ui::window::options::AppPrefs {
+                            savetime_min: self.savetime_min,
+                            backup_on_save: self.backup_on_save,
+                        },
                         self.double_click_block_refedit,
                         self.double_click_block_attedit,
                         self.cursor_type,