feat(app,android,ios): swipe gestures, touch lock and sleep timer

On Android and iOS a swipe up or down on the left half of the picture
sets the brightness and on the right half the volume; a sideways swipe
seeks, showing where it will land and seeking only when the finger
lifts. Swipes that start at the top or bottom edge are left to the
system. Android turns the device's media volume; iOS lets no app do
that, so there the swipe turns the player's own, as its slider does.
Brightness set by a swipe is handed back when the player goes.

Lock touch, in the player's menu, makes the picture ignore touches and
the back gesture; a tap brings up Unlock for a few seconds.

The sleep timer pauses after 30 or 60 minutes, or stops at the end of
the episode playing instead of going on to the next, with no up-next
card. On the desktop it is a panel like the others, behind a moon
button that shows what is left; on Android and iOS it lives with the
player host, so it keeps running with the screen off.

docs/IOS.md

@@ -94,6 +94,8 @@ Each of these is the platform's doing, not a choice to diverge.
   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.
+- **The volume swipe turns the player's own volume**, the one its slider sets.
+  iOS lets no app set the device's media volume, which Android's swipe turns.
 - **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