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.

frontend/src/components/ui/dialogFocus.ts

@@ -1,4 +1,4 @@
-import { useEffect, useState, type RefObject } from "react";
+import { useEffect, useRef, useState, type RefObject } from "react";
 
 // The focus half of the dialog contract, in ONE place. `ui/Modal` owns it for every dialog that goes
 // through the shell; the onboarding wizard renders its own shell (a bare `ui/Overlay`, because it is
@@ -54,19 +54,47 @@ export function useDialogChrome(scopeRef: RefObject<HTMLElement | null>): void {
   // later (in the effect) would capture a now-inside element that unmounts on close, stranding focus
   // on <body>. useState initializer runs exactly once, on first render.
   const [restoreTo] = useState(() => document.activeElement as HTMLElement | null);
+  // What the dialog is focused ON, so a RE-mount can put the focus back where it was rather than on
+  // the first control. React 18's StrictMode runs every effect mount→cleanup→mount in development:
+  // the cleanup hands focus back to the trigger, and the second mount then found the focus outside
+  // the dialog and moved it to the first focusable — taking it off the `autoFocus` field a beat
+  // after the browser had put it there. Measured in Chrome: the command palette opened with the
+  // focus on its ✕ in dev and on its search field in prod, from the same code (round 5).
+  const focusedRef = useRef<HTMLElement | null>(null);
+  // Whether the dialog still holds the focus. Read at cleanup rather than computed there: by the
+  // time a passive cleanup runs on a real unmount the panel is already out of the document and the
+  // focus has fallen to <body>, so "does the panel contain activeElement" answers false for both a
+  // close and a user who clicked away — and those two want opposite things.
+  const heldRef = useRef(true);
   useEffect(() => {
     const prev = document.body.style.overflow;
     document.body.style.overflow = "hidden";
     // Initial focus: unless an autoFocus child already placed it inside (e.g. a rename input), move
-    // focus to the first control so the dialog opens focused, not stranded on the background.
+    // focus to where this dialog was focused before — or, the first time, to the first control, so
+    // the dialog opens focused rather than stranded on the background.
     const scope = scopeRef.current;
     if (scope && !scope.contains(document.activeElement)) {
-      (focusablesIn(scope)[0] ?? scope).focus();
+      const back = focusedRef.current;
+      const target =
+        (back?.isConnected && scope.contains(back) ? back : null) ??
+        focusablesIn(scope)[0] ??
+        scope;
+      target.focus();
     }
+    const onFocusIn = () => {
+      const inside = scopeRef.current?.contains(document.activeElement) ?? false;
+      heldRef.current = inside;
+      if (inside) focusedRef.current = document.activeElement as HTMLElement;
+    };
+    onFocusIn();
+    document.addEventListener("focusin", onFocusIn);
     return () => {
+      document.removeEventListener("focusin", onFocusIn);
       document.body.style.overflow = prev;
-      // Restore focus to the trigger, if it's still in the document and focusable.
-      if (restoreTo && restoreTo.isConnected) restoreTo.focus();
+      // Restore focus to the trigger, if it's still in the document and focusable — and only if the
+      // dialog still held the focus. Once the user has moved it elsewhere, taking it back is the
+      // rudest thing a closing dialog can do.
+      if (heldRef.current && restoreTo && restoreTo.isConnected) restoreTo.focus();
     };
   }, [restoreTo, scopeRef]);
 }