v0.26.0: A repeated page folds down instead of disappearing

A repeated page folds down instead of disappearing

v0.25.0 removed the pages that are not the story — the credit page, the advert — from the chapter you were
reading. That was the wrong shape, and reading with it for a day made the reason plain: a chapter was quietly
shorter than it really was, a floating chip announced the fact at every chapter start whether you cared or
not, and there was no way to see what was going to be removed before it went.

Now the page stays exactly where it is, drawn as a thin band of itself with a label. You scroll past it in an
instant, or tap it to open it in place and tap collapse to fold it away again. The band is a slice of the
real page, so you can see it is the credit page rather than take our word for it — which is the part that was
missing. The floating chip is gone: the notice sits where the page is, which is both harder to miss and
impossible to mistake for a comment about something else.

Repeated pages in reader settings now offers Show all, Collapse (the default) and Hide. Hide is
the old behaviour for anyone who wants the page gone outright, chip and all. Reading page-by-page rather than
scrolling, Collapse still removes — a slide is one whole page wide, so there is no room for a band, and in
that mode an unwanted page costs a swipe rather than a scroll anyway.

Four bugs that removal had been causing

Putting the page back in the list the reader counts with fixed a set of failures that all had the same root:
while pages were being removed, a position in the chapter and a page number were two different things, and
several places assumed they were the same.

• Resume and saved Moments landed late. Opening a Moment saved on page 3 took you to page 4 — one page
  further on for every repeated page earlier in the chapter. Silently, because arriving a page on is
  indistinguishable from having read that far.
• The chapter divider vanished when a chapter opened on a credit page, taking the "Up Next" heading with
  it — and because the same marker keeps a chapter's first page unpaired, every double-page spread in that
  chapter was shifted by one.
• Tapping a dimmed tile in the page grid scrolled to the top of the entire library instead of to the page
  you tapped.
• A chapter that was entirely furniture disappeared, and continuous reading walked from the chapter
  before it to the chapter after with nothing in between.

The reading flow is now built in one place, web/lib/readerFlow.ts, where it can be tested — which is why
these were reachable at all. Each has a test that fails when the old behaviour is put back.