feat(app,android,ios): full-bleed hero carousel with ambient colour

The library's banner now runs edge to edge and turns through up to five
titles: the one watched last, then titles with a wide picture, starting
from a different one each day. It moves on every eight seconds, holds
still under the pointer, and has dots to pick a title. A part-watched
title shows how far in the viewer is under its buttons.

The veils over the banner and over a title page's band are tinted with
the picture's own colour. The weighting and the veil live in
pstr_core::appearance (mood, mood_veil), so the desktop and both phones,
which reach them through the bridge, land on the same colour.

On Android the banner is a HorizontalPager under a clear bar that fills
in as it scrolls away; iOS uses a paged TabView the same way.

docs/DESIGN-TOKENS.md

@@ -63,9 +63,23 @@ this module, and keep Material's sizes. Nothing in the app draws with them.
 buttons, the tab pill. **4** (`BADGE`) for a badge over artwork. The desktop
 also has **3** (`BAR`) for progress bars and edge strips, **6** (`SM`) for a
 small control inside something else, and **12** (`LG`) for what floats — row
-cards, dialogs, toasts, the hero. **16** for an Android bottom sheet, which has
+cards, dialogs, toasts. The library's banner is square: it runs edge to edge. **16** for an Android bottom sheet, which has
 no desktop counterpart.
 
+## A picture's colour
+
+The veils over a title's backdrop — the library's banner, the top of a title's
+page — are the page colour tinted by the picture under them, so each title
+carries its own light. All three clients get the same colour from the same
+code: `pstr_core::appearance::mood` averages the picture, squeezed to a few
+hundred pixels, counting a saturated pixel up to five times a grey one
+(`MOOD_CHROMA_WEIGHT`), and `mood_veil` turns that into the veil. On a dark
+palette the mood is darkened until its brightest channel is 60 (`MOOD_DEPTH`)
+and the page moves 55% of the way to it (`MOOD_DARK`); a light palette moves
+12% (`MOOD_LIGHT`). The phones reach both through the bridge, as
+`pictureMood` and `moodVeil`. The fade at the bottom stays the page colour,
+because that is what it ends in.
+
 ## Spacing
 
 The desktop spaces things from one scale, `theme::space`: 2, 4, 6, 8, 12, 16,