fix(player): a step onto an unplayable video carries on the way it went

Previous onto an embed-blocked or removed video skipped forward again (the
auto-advance direction), straight back to the item just left. The player now
remembers the direction of the step that reached the item and the videos
YouTube refused this session: an error carries on that way, Previous / Next
pass over known-dead items without loading them, and go dark when only those
are left. A step of your own skips with auto-advance off too.

frontend/src/features/player/playerModalControls.tsx

@@ -27,6 +27,8 @@ export function PlayerTransportControls({
   variant,
   index,
   queueLength,
+  canPrev,
+  canNext,
   autoMode,
   loopMode,
   onPrev,
@@ -37,6 +39,9 @@ export function PlayerTransportControls({
   variant: "docked" | "modal";
   index: number;
   queueLength: number;
+  /** Whether there is a playable item that way (one YouTube refused is passed over). */
+  canPrev: boolean;
+  canNext: boolean;
   autoMode: AutoMode;
   loopMode: LoopMode;
   onPrev: () => void;
@@ -70,7 +75,7 @@ export function PlayerTransportControls({
       name: t("player.previous"),
       title: `${t("player.previous")} · Shift+←`,
       onClick: onPrev,
-      disabled: index === 0,
+      disabled: !canPrev,
       labelFrom: "sm",
     },
     {
@@ -79,7 +84,7 @@ export function PlayerTransportControls({
       name: t("player.next"),
       title: `${t("player.next")} · Shift+→`,
       onClick: onNext,
-      disabled: index === queueLength - 1,
+      disabled: !canNext,
       labelFrom: "sm",
       trailing: true,
     },