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>

frontend/src/types.ts

@@ -105,6 +105,8 @@ export interface Chapter {
   id: string;
   notebookId: string;
   name: string;
+  /** `#rrggbb` for its icon, so a shelf of chapters can be told apart. */
+  color?: string | null;
   sortOrder: number;
   /** Set while a chapter created offline has not reached the server yet. */
   _pendingCreate?: boolean;
@@ -129,8 +131,18 @@ export interface Notebook {
 /** How the note body is set. Presentation only — the export is unaffected. */
 export type NoteFont = "serif" | "sans" | "mono";
 
+/**
+ * How a chapter opens.
+ *
+ * "column" puts its notes in a further column beside the list, the way OneNote
+ * lays out its pages; "inline" unfolds them under the chapter's own row. A
+ * phone has room for neither and fills the screen either way, with a way back.
+ */
+export type ChapterView = "column" | "inline";
+
 export interface UserPreferences {
   noteFont: NoteFont;
+  chapterView?: ChapterView;
 }
 
 export interface User {