feat(cast): music on the TV, the music bar as its remote

A cast button in Music's toolbar and in the music bar. With a television
chosen, each decrypted track goes to the relay with its cover — found as the
album card finds it — and plays there as music with its title, artist and
album; the bar's play, pause, seek, previous and next drive the receiver, its
clock is the receiver's, and the end of a track there moves the queue on.
A film or a photo taking the television pauses the bar; stopping the cast
carries the track on locally.

Photos and tracks now share one path: a whole file sent to the relay in
pieces (binary frames on Android, written to disk there), served at /file
with byte ranges and its cover at /cover, and loaded as what the relay says
it is. cast:image is gone; cast:chromecast:seek is new.

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

1 files changed, 17 insertions, 10 deletions

diff --git a/docs/USERGUIDE.md b/docs/USERGUIDE.md
index 3b2f16c..97ea679 100644
--- a/docs/USERGUIDE.md
+++ b/docs/USERGUIDE.md
@@ -394,11 +394,11 @@ resume.
 
 ### Casting to a TV
 
-**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
+**The desktop and Android applications.** Films, photos and music can be sent
+to a cast-capable TV or dongle on the same network. The cast button is in the
+Videos and Music toolbars, at the top of Photos, in an album's bar, in the photo
+viewer and in the music bar — in a group and in Search alike — and *Cast to
+device* is in the video 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.
 
@@ -407,12 +407,19 @@ 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.
+album's bar opens its first photo. Music plays on it too, with the cover, title
+and artist on the screen: the music bar becomes its remote — play, pause, the
+slider, previous and next — and the queue moves on when the TV reaches the end
+of a track. Choosing the TV in the middle of a song carries it on there; *Stop
+casting* carries it on here. On a phone, a film goes on playing silently on the
+phone while the TV shows it, and the screen can be turned off, as it can while
+music plays on the TV.
 
 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.
+what is playing, not the application. It plays it in the TV's standard cast
+player, which shows its own name, *Default Media Receiver*, on the screen; a
+player of MeshBay's own is not built.
 
 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
@@ -993,8 +1000,8 @@ 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, for films and photos. Music is not cast yet. Support for other
-  TV protocols is designed but not built.
+  applications, for films, photos and music. 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.