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

TASKS.md

@@ -250,3 +250,5 @@ Per-component lines live in `tasks/`. Tags there:
 | T21a | 1 | FAIL | Enter kept a selection; undo of a split left focus on a gone field; italic inside bold ate the bold; keys went into merged cells; undo kept a changed kind's highlighter |
 | T21a | 2 | FAIL | undo of typing lost the selection it replaced; Up from below a table landed in a hidden merged cell |
 | T21a | 3 | FAIL | undo after a delete or a paste lost the selection they replaced; fixed after the cap by recording the replaced selection where every TextInput edit passes, verified by a red-green test, no fourth review |
+| T21b | 1 | FAIL | a page zoomed wider than the view sat past its left edge; a new hit at the same index kept the old page in view; a hit current from the first frame only scrolled its page to the top; the chapter list did not scroll; Escape could not leave zen turned on from a button |
+| T21c | 1 | FAIL | the gallery's page properties dropped a new due date and tags |