fix(ui,plex): a dialog keeps the focus its child asked for, and a hopeless segment fails at once

Both found by UAT in a real browser, after the tests for them were already green — which is the
whole reason this repo settles focus and timing claims by running them.

- **The autoFocus fix worked, and then the dialog took the focus back.** React 18's StrictMode runs
  every effect mount→cleanup→mount in development: the cleanup handed focus back to the trigger, and
  the second mount, finding the focus outside the dialog, moved it to the first focusable — off the
  `autoFocus` field a beat after the browser had put it there. Chrome measured it: INPUT (commitMount)
  → BODY (cleanup) → ✕ (re-mount). The effect is idempotent now — it remembers what the dialog was
  focused on and restores THAT on a re-mount — and the close-time restore is conditional on the
  dialog still holding the focus, which it tracks as it changes rather than computing at cleanup,
  where the panel is already out of the document. A dialog no longer yanks focus back from wherever
  the user has since moved it.

- **Passing ffmpeg's handle to the segment wait only shortened the FINISHED case.** Measured against
  the running localdev API: a request for `seg_99999.ts` on a live session still took 20 195 ms,
  because that ffmpeg has not exited and the wait has no other reason to stop. ffmpeg writes segments
  in order, so a segment whose predecessor does not exist is not the next one and cannot arrive —
  and that is knowable at once. Same request now: 45 ms.

Verified on localdev against the real library (Westworld S01E06, remux, multi-audio): Mod+K opens the
command palette with the focus in its search field and typing reaches it; playback runs on the new
session-scoped HLS URL; a request naming a stale session id is a 404; and a viewer paused for longer
than the finished-session rule keeps their 2.4 GB session across two reaper passes — the progress
checkpoint touches it — and resumes cleanly.

vitest 1009, pytest 1127.

backend/app/routes/plex/playback.py

@@ -656,6 +656,12 @@ def stream_hls_file(
     if s is None or s.id != session_id:
         raise HTTPException(status_code=404, detail="No active playback session")
     path = s.dir / filename
+    # A segment beyond the generated region can never arrive: ffmpeg writes them in order, so if the
+    # predecessor is not there this one is not next. Answering now instead of waiting it out is what
+    # keeps a seek past the produced region — or a deliberate flood of hopeless names — from parking
+    # threadpool workers and their DB connections (see `segment_may_arrive`).
+    if not plex_stream.segment_may_arrive(s.dir, filename):
+        raise HTTPException(status_code=404, detail="Not available yet")
     timeout = 30 if ext == ".m3u8" else 20
     # `proc` so a session whose ffmpeg has already exited answers at once instead of polling out the
     # whole timeout: this is a SYNC route on the ~40-slot threadpool, holding a pooled DB connection