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/mod.rs

@@ -2007,6 +2007,10 @@ pub enum Message {
     ShiftToAddToggled(bool),
     /// Toggle press-and-drag drawing a rectangle instead of a lasso (PICKDRAG).
     PickDragRectToggled(bool),
+    /// Change the automatic-save interval in minutes; 0 disables it (SAVETIME).
+    SaveTimeChanged(i32),
+    /// Toggle keeping a `.bak` copy when overwriting a drawing (ISAVEBAK).
+    BackupOnSaveChanged(bool),
     /// Restore Model Space display/canvas appearance to defaults.
     RestoreModelSpaceDisplayDefaults,
     /// Restore Selection visual effect settings to defaults.