feat(android,ios): parallax title page with a sticky bar

On a phone the backdrop at the top of a title page now moves at half
the speed of the page scrolling over it, and the back button sits in a
bar that fills in with the title's name once the backdrop has gone, so
a long episode list still says whose it is. The fade into the page is
tinted with the backdrop's colour, as on the library's banner.

iOS keeps its slide in from the edge: the zoom transition needs a
NavigationStack, and the title page is shown over the library by hand.

docs/IOS.md

@@ -90,6 +90,10 @@ Each of these is the platform's doing, not a choice to diverge.
   a tap that missed. `PressableStyle` and `tapAndHold` (`UI/Theme/Accent.swift`)
   stand in for the ripple, and a settings page goes back by an edge strip of its
   own, as a title page does, where Android has the system back.
+- **No zoom into a title page.** iOS 18's `.navigationTransition(.zoom)` only
+  runs inside a `NavigationStack`, and the title page is shown over the library
+  by hand, so it keeps its slide in from the edge — Android's shared poster has
+  no counterpart here.
 - **Nothing is backed up**, as on Android (`allowBackup="false"`). The share
   secrets are this-device-only Keychain items, so a restored share list would
   arrive without them, and a restored `sync.json` would give two devices one sync

docs/USER_GUIDE.md

@@ -67,7 +67,9 @@ containers.
 
 ## A title's page
 
-Click a title to open its page. **Esc** goes back.
+Click a title to open its page. **Esc** goes back. On a phone, the picture at the top slides
+away more slowly than the page, and once it has gone a bar with the title's name stays at the
+top.
 
 - **Resume S01E03 · 12:04** continues where you stopped. **Start over** plays the episode from
   the beginning.