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, 18 insertions, 7 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/platform.js b/packages/meshbay-hub/src/meshbay_hub/static/platform.js
index 3a0c628..28817c7 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/platform.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/platform.js
@@ -460,14 +460,16 @@ export const cast = {
     if (!bridge || !bridge.cast || !bridge.cast.subtitle) return null;
     return bridge.cast.subtitle(sub);
   },
-  // A photo for the receiver: `{ bytes, type }`, already scaled to the
-  // television. Absent from an older shell, so callers check `canShowImages`.
-  get canShowImages() {
-    return Boolean(bridge && bridge.cast && bridge.cast.image);
+  // A whole file for the receiver — a photo already scaled to the television,
+  // a music track — as `{ bytes, type, cover?: { bytes, type }, title?,
+  // artist?, album? }`; answers `{ url }` to load. Absent from an older
+  // shell, so callers check `canSendFiles`.
+  get canSendFiles() {
+    return Boolean(bridge && bridge.cast && bridge.cast.file);
   },
-  async image(img) {
-    if (!this.canShowImages) return null;
-    return bridge.cast.image(img);
+  async file(f) {
+    if (!this.canSendFiles) return null;
+    return bridge.cast.file(f);
   },
   async finish() {
     if (!bridge || !bridge.cast) return false;
@@ -529,6 +531,15 @@ export const cast = {
     if (!this.canControl) return null;
     return bridge.cast.chromecastPlay();
   },
+  // Seeking inside a file the receiver plays (a track); a film seeks by
+  // restarting the relay instead.
+  get canSeek() {
+    return Boolean(bridge && bridge.cast && bridge.cast.chromecastSeek);
+  },
+  async chromecastSeek(seconds) {
+    if (!this.canSeek) return null;
+    return bridge.cast.chromecastSeek(seconds);
+  },
 };
 
 /**