Chapters as rows, in two views, in their own colours

Three things Marc asked for, and one of them turned out to be the hinge
the other two hang from.

**A chapter is a row like any other.** Same grip, same drag, same place
in a hand-made arrangement as a note or a separator. That needed notes
and chapters numbered from *one* counter: numbered separately they are
two sequences that cannot be interleaved again, and a chapter dragged
between two notes springs back to the top. Positions therefore have to be
named rather than read off an array index, so POST /notes/reorder now
takes `positions: [{id, order}]` beside the older `ids`, and chapters
have a reorder route of their own.

**Two ways for a chapter to open**, chosen per account beside the
typeface, for the same reason that one lives there: it is a choice about
how your own notes are laid out and it should follow you to the phone.
Beside the list, its notes in a further column the way OneNote lays out
pages — the notebook's own list carrying on to the left, chapter rows and
all. Or unfolded under the chapter's own row, indented against a rule in
its colour. A phone has room for neither and fills the list instead, with
a way back, which is what it did before there was a choice.

**Its own colour**, from eight named ones rather than the full palette:
the point of chapters is that they are different places, and twenty
near-identical blues do not help anyone tell them apart.

The colour is validated on the way in — it is written by a browser, so
only a plain #rrggbb gets through and anything else becomes no colour.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -8,6 +8,26 @@ not from the output of `docker push`.
 
 ---
 
+## Unreleased
+
+**Added**
+- **Two ways for a chapter to open**, chosen per account: its notes in a further
+  column beside the list, the way OneNote lays out its pages, or unfolded under
+  the chapter's own row. A phone has room for neither and fills the list, with a
+  way back.
+- **A chapter can be given its own colour**, since the point of having them is
+  that they are different places.
+
+**Changed**
+- **Chapters sit at the same level as notes and separators** — same grip, same
+  drag, same place in a hand-made arrangement. Notes and chapters are numbered
+  from one counter now; separately, the two sequences could not be interleaved
+  and a chapter dragged between two notes would spring back. `POST
+  /notes/reorder` takes named positions beside the older list of ids, and
+  chapters have a reorder route of their own.
+
+---
+
 ## 2026-08-12 — Moving in one press
 
 **Images** — web `sha256:3700247c…`, server unchanged at `sha256:181d77be…`