T21b and T21c: Documents, reading and knowledge

- DocumentViewer: pages the app drew on a desk, the page under the view's middle
  counted, zoom steps and fit, thumbnails, notes pinned where a press lands, and
  the owner's FindWidget over the pages, hits lit and the current one revealed
- PageThumbnailList, EpubReader (chapters, type size, progress), PrintPreview
- MarkdownRenderer feeds TableOfContents through sections and outline
- Footnote, TooltipTerm, Glossary, ReadingProgress, ZenMode
- Backlinks, Favorites, TrashBin, PageCover, PageIcon, TemplatePicker,
  VersionHistory with PageHistoryDiff
- editor::FindWidget shows the replace fold and option toggles only with handlers
- block editor tests move to documents/blocks
- icons: a-arrow-down, a-arrow-up
- gallery: reading and knowledge sections; scripted find, changes and zen

src/theme/tokens.rs

@@ -279,6 +279,16 @@ impl Theme {
         px_to_rems(20.0)
     }
 
+    /// A page's cover band.
+    pub fn page_cover(&self) -> Rems {
+        px_to_rems(208.0)
+    }
+
+    /// A page's icon, an emoji set large.
+    pub fn page_icon(&self) -> Rems {
+        px_to_rems(64.0)
+    }
+
     /// A picture's height in a skeleton card.
     pub fn skeleton_media(&self) -> Rems {
         px_to_rems(128.0)