fix(plex): a link too slow for the stream resumes in place, and says so

A request hls.js got no answer to in time (no status) was taken for a server
away: a new session, the same first segment lost again, a restart every half
minute that never ended, since every session answer reset the away window.
Measured on prod: a work network at ~4 Mbit/s against 11-16 MB remux segments.

A timeout now resumes loading in the same session; the away window ends only
when media arrives (or a direct stream answers), and runs out in a 'too slow'
error. A segment gets 30 s to its first byte. After 5 s of stalling on a link
measured slower than the stream, the player says so with both rates.

frontend/src/modules/plex/PlexPlayer.tsx

@@ -70,7 +70,9 @@ import {
   usePlayerVolume,
   usePlexProgress,
   usePlexTracks,
+  useSlowLink,
   useWallClock,
+  slowLinkRates,
 } from "../../lib/plexPlayerHooks";
 import { canFullscreenElements, fullscreenElement } from "../../lib/fullscreen";
 import {
@@ -101,7 +103,7 @@ import { MenuCheck } from "../../components/ui/MenuCheck";
 import { PopoverItem, PopoverSurface } from "../../components/ui/AnchoredPopover";
 import { closeThenAct, useAnchoredPopover } from "../../components/ui/anchoredPopoverHooks";
 import { Slider } from "../../components/ui/NativeControls";
-import { formatUnit } from "../../lib/format";
+import { formatMbps, formatUnit } from "../../lib/format";
 import { trackLabel } from "../../lib/vocab";
 import { msg } from "../../lib/notifications";
 
@@ -286,9 +288,16 @@ export default function PlexPlayer({
       ? t("plex.player.errTranscode")
       : engineError === "notFound"
         ? t("plex.player.errNotFound")
-        : engineError
-          ? t("plex.player.errGeneric")
-          : null;
+        : engineError === "slow"
+          ? t("plex.player.errSlow")
+          : engineError
+            ? t("plex.player.errGeneric")
+            : null;
+  // A stall that lasts, on a link measured slower than the stream: said under the spinner, with both
+  // numbers, so the viewer can tell their connection from a broken player.
+  const slowRates = useSlowLink((!ready || buffering) && !loadError, () =>
+    slowLinkRates(fragMeterRef.current.bitrate(), fragMeterRef.current.arrival()),
+  );
   // Refs for values read inside stable callbacks / the keyboard handler (avoid stale closures).
   const barAutoHideRef = useLatest(prefs.barAutoHide);
   // The two language prefs are read only at ITEM LOAD (the detail effect below), so they go through
@@ -1326,7 +1335,17 @@ export default function PlexPlayer({
               {/* Announce the stall to assistive tech. Initial load shows visible text; a mid-playback
                   rebuffer/seek (ready, no error) has no visible label, so give it an sr-only one via the
                   aria-live region so a screen reader distinguishes "buffering" from a frozen frame. */}
-              {!ready ? (
+              {slowRates ? (
+                <>
+                  <span>{t("plex.player.slowLink")}</span>
+                  <span className="text-xs text-white/60">
+                    {t("plex.player.slowLinkRates", {
+                      need: formatMbps(slowRates.need, i18n.language),
+                      got: formatMbps(slowRates.got, i18n.language),
+                    })}
+                  </span>
+                </>
+              ) : !ready ? (
                 <span>
                   {t(sessionIsTranscode ? "plex.player.preparing" : "plex.player.loading")}
                 </span>