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/USER_GUIDE.md

@@ -13,7 +13,11 @@ The app has five pages, in the bar at the top: **Library**, **History**, **Share
 The library shows every title from every share in one place. Two shares that hold the same show
 show it once.
 
-- **The banner** at the top shows the title you watched last, with **Resume** and **More info**.
+- **The banner** across the top shows the title you watched last first, with **Resume**, how far
+  into it you are and **More info**. It then turns through up to four more titles that have a wide
+  picture, a different set each day, moving on every eight seconds. It holds still while the
+  pointer is over it; on a phone, swipe it or tap a dot under it to move on yourself. Its shading
+  takes on the colours of the picture behind it, and so does the top of a title's page.
 - **Continue watching** has the episode you stopped in for each show, or the next one when you
   finished an episode. To take a show off this row, right-click it and choose **Remove from
   Continue watching**.