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

TASKS.md

@@ -69,8 +69,8 @@ Per-component lines live in `tasks/`. Tags there:
 - [x] T19 Git
 - [x] T20 Debug
 - [x] T21a Documents · Editing — `tasks/ch21-30.md`
-- [ ] T21b Documents · Reading
-- [ ] T21c Documents · Knowledge
+- [x] T21b Documents · Reading
+- [x] T21c Documents · Knowledge
 - [ ] T22 Collaboration
 - [ ] T23a AI Chat · Messages
 - [ ] T23b AI Chat · Citations