fix(player): give every transcode its own play session, and stop the one it replaces

Switching bitrate mid-film stalled playback. The server served the new
playlist and then rejected its segments: 400 on hls1/main/0.ts, six times
over 25 seconds, never recovering, while the UI logged "Streaming quality
changed" as if nothing were wrong.

Jellyfin keys a transcode job by device and play session. Every stream URL
this app built carried the same hardcoded DeviceId and no PlaySessionId at
all, so the second stream for an item was indistinguishable from the first
and nothing ever stopped the old ffmpeg. Re-opening a stream is not rare —
a quality switch, a transcoded seek and an audio-track switch all do it.
Replayed against the server, a second stream opened for a live job's item
alternates per attempt between serving bytes and 400ing, which is why it
read as flaky rather than broken.

begin_video_play_session mints a session id per open and reports the one it
supersedes; the URL builder stops that job (DELETE /Videos/ActiveEncodings,
un-retried — a slow stop must not delay playback) before returning. Putting
it in the builder rather than in each caller covers every re-open path by
construction. adopt_video_play_session takes ownership of the job the server
starts itself when PlaybackInfo answers with a TranscodingUrl: without it the
first switch on a stream has nothing to stop and collides with what is
playing.

Two client faults made the same incident worse and go with it:

- The fatal-HLS-error handler added the transcode seek offset to a position
  that already included it. Past roughly the halfway mark of a film the
  doubled value cleared the "near end" threshold, so any transient network
  error was reported as end-of-stream and autoplay skipped to the next item
  — precisely when a quality switch had just made the offset large. The
  decision now lives in hlsRecovery.ts, against the absolute position.
- The HTML5 reload primitive resolved on its own canplay timeout, so a
  reload the server never served reported success. The picker showed a
  quality that was not playing and the caller had nothing to revert.

TRACES: UR-074, UR-004 | DR-177 | UT-173, UT-174, UT-175

src/lib/api/bindings.ts

@@ -2235,20 +2235,7 @@ type: string;
 /**
  * Provider-neutral stream classification — replaces `stream_type`.
  */
-kind?: StreamKind; codec?: string | null; language?: string | null; displayTitle?: string | null; index: number; isDefault: boolean; isForced: boolean; 
-/**
- * Whether this stream can reach the app as a sidecar it renders itself.
- * 
- * `None` for anything that is not a subtitle — the question does not apply,
- * and `false` there would read like a verdict. For a subtitle it is the
- * difference between a track the app can draw and one only the server could
- * have shown, by burning it into the picture (DR-176) — which this app never
- * asks it to do. The vocabulary of *which formats those are* stays in Rust;
- * the frontend only reads the answer.
- * 
- * TRACES: UR-020 | DR-176 | UT-168
- */
-supportsExternalDelivery?: boolean | null }
+kind?: StreamKind; codec?: string | null; language?: string | null; displayTitle?: string | null; index: number; isDefault: boolean; isForced: boolean }
 export type MediaType = "audio" | "video"
 /**
  * Lightweight media item for merged playback state