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>

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