feat(cast): photos on the TV, and a television chosen once for the session

A cast button in Videos' toolbar, at the top of Photos, in an album's bar and
in the lightbox, in a group and in Search alike. A television chosen there is
kept for the session: a film opened plays on it with the player as its remote
from the start, and a photo opened in the lightbox is shown on it, scaled to
1920x1080, upright, as JPEG. The lightbox gains a slideshow.

The relay serves one photo at /image behind the stream's token, on the desktop
and on Android; the shell, not the page, decides that the receiver loads it as
a picture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 24 insertions, 11 deletions

diff --git a/docs/USERGUIDE.md b/docs/USERGUIDE.md
index d6eab26..3b2f16c 100644
--- a/docs/USERGUIDE.md
+++ b/docs/USERGUIDE.md
@@ -352,7 +352,9 @@ An album browser and a player, over the folders the operator chose for it.
 ### Photos
 
 An album browser over the folders the operator chose for it, where **an album
-is a folder**. Thumbnails come from the node, already rotated correctly.
+is a folder**. Thumbnails come from the node, already rotated correctly. A
+photo opens full size, with a slideshow button that moves on every five
+seconds and stops at the album's last photo.
 
 **Location data is never shown.** Photos shows when a picture was taken and
 what took it, and no coordinates anywhere. Worth knowing, though: the
@@ -392,15 +394,25 @@ resume.
 
 ### Casting to a TV
 
-**The desktop and Android applications.** A film playing in the application can
-be sent to a cast-capable TV or dongle on the same network: *Cast to device* in
-the player, pick one from the list. On a phone, the film goes on playing silently
-on the phone while the TV shows it, and the screen can be turned off. While a
-TV plays the film, the player becomes its remote: the position shown is the
-TV's, with play/pause, 30-second jumps and a slider to go anywhere in the film.
-Devices appear as they answer, usually within a couple
-of seconds; the search carries on for a few more, for a TV that is still
-waking up.
+**The desktop and Android applications.** Films and photos can be sent to a
+cast-capable TV or dongle on the same network. The cast button is in the
+Videos toolbar, at the top of Photos, in an album's bar and in the photo
+viewer — in a group and in Search alike — and *Cast to device* is in the
+player. Devices appear as
+they answer, usually within a couple of seconds; the search carries on for a
+few more, for a TV that is still waking up.
+
+**A TV, once chosen, stays chosen** until *Stop casting*, from any cast button
+or from the player. Every film opened plays on it, and the player opens as its
+remote: the position shown is the TV's, with play/pause, 30-second jumps and a
+slider to go anywhere in the film. Every photo opened in the viewer is shown on
+it, and a slideshow pages the TV along with the screen. Choosing a TV from an
+album's bar opens its first photo. On a phone, a film goes on playing silently
+on the phone while the TV shows it, and the screen can be turned off.
+
+Photos are sent scaled to the TV's screen, upright, as an ordinary picture.
+The menus, covers and summaries stay on your phone or computer: the TV shows
+what is playing, not the application.
 
 The application decrypts the film and relays it to the TV itself, over your LAN.
 The TV is not a group member and holds no key — which is also why the relay's
@@ -981,7 +993,8 @@ Better to know now than to go looking for it:
 - **Hubs do not talk to each other yet.** Everyone in a group needs an account
   on the same hub.
 - **Casting reaches Chromecast devices** from the desktop and Android
-  applications. Support for other TV protocols is designed but not built.
+  applications, for films and photos. Music is not cast yet. Support for other
+  TV protocols is designed but not built.
 - **Some subtitle tracks cannot be shown** — the ones stored as images rather
   than text, roughly one embedded track in five. Displaying them would need
   text recognition.