webclient: a viewer for every content type, and a message sent mid-turn waits its course ...

The lightbox is now one dialog for every file the artifacts panel and the
content cards can show: audio and video players on the kit's own controls, a
text/source view with highlight.js, and a three.js viewer that replaced the
standalone stl.html frame with a model one. highlight.js emits .hljs-* classes
and ships no theme, so until now every highlighted block rendered in one flat
colour — app.scss now carries the palette.

While a turn runs, the send button no longer turns into Stop and vanishes the
draft with it: the message goes into the socket and is marked "queued" until
the stream_start of the turn that consumes it, the write-now-run-next model
navi-code uses. Send and Stop sit side by side, and the queue chip takes the
larger of the local and the server's counts.

The socket keeps its own liveness: a ping every 20 s, a watchdog that reopens a
socket silent for 45 s or stuck in CONNECTING, and a reconnect on resume from
background, network return or bfcache. On reopen it drops a half-written
message left by the dead socket, so the session_sync that follows is not hidden
behind a stream that will never finish.

Add artifact source previews