T21b and T21c round 1 fixes

- a page centers only while it fits the view; a wider one starts in reach
- the reveal runs after the pages' prepaint and keys on the hit itself, so a
  new hit at the same index and a hit current from the first frame come into view
- the chapter list scrolls
- zen takes focus when turned on from outside, so Escape leaves it, and hands
  focus back
- the gallery keeps a page's due date and tags

src/documents/epub.rs

@@ -104,8 +104,10 @@ impl RenderOnce for EpubReader {
         let shown = &self.chapters[chapter];
         let listed = contents.then(|| {
             div()
+                .id((self.id.clone(), "chapters"))
                 .flex_none()
                 .w(theme.sidebar_width(false))
+                .overflow_y_scroll()
                 .flex()
                 .flex_col()
                 .gap_0p5()