fix(player): short-video auto-watch + player cleanups

BUG (YB1): the auto-watch checkpoint marked a video watched once position
crossed `duration - FINISH_MARGIN` (10s). For clips shorter than ~10s that
threshold is negative, and for ~11-20s clips it's only a few seconds in, so the
5s checkpoint tick marked short videos watched almost immediately (clearing their
resume position). Cap the margin at half the clip: Math.min(FINISH_MARGIN, dur/2).

Cleanups (behavior-neutral):
- Extract format.formatDate(iso, lang) — the day/month/year toLocaleDateString
  option object was duplicated verbatim in PlayerModal (fullDate) and VideoCard.
  (ChannelPage's "joined" is month/year only, so it's left as-is.)
- The in-bar prev/next buttons now reuse goPrev/goNext instead of re-implementing
  the step + bounds inline (they already exist for the side arrows + keyboard).
- Memoize savedPrefs so the ['me'] cache read + spread runs once, not on every
  render (it only seeds the initial autoMode/loopMode state).

tsc green, knip clean, localdev boots healthy.

frontend/src/components/PlayerModal.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef, useState } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { createPortal } from "react-dom";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
@@ -7,7 +7,7 @@ import Avatar from "./Avatar";
 import AddToPlaylist from "./AddToPlaylist";
 import DownloadButton from "./DownloadButton";
 import { api, type Video } from "../lib/api";
-import { channelYouTubeUrl, formatDuration, formatViews, relativeTime } from "../lib/format";
+import { channelYouTubeUrl, formatDate, formatDuration, formatViews, relativeTime } from "../lib/format";
 import { renderDescription } from "../lib/descriptionLinks";
 import { useBackToClose } from "../lib/history";
 
@@ -84,14 +84,7 @@ export default function PlayerModal({
   // non-embeddable still surface the existing playback-error overlay when reached.
   const [queue] = useState(() => queueProp);
   // Precise upload date shown inline after the relative time (e.g. "9 yr ago · 12 Jan 2017").
-  const fullDate = (d: string | null | undefined) =>
-    d
-      ? new Date(d).toLocaleDateString(i18n.language, {
-          year: "numeric",
-          month: "short",
-          day: "numeric",
-        })
-      : "";
+  const fullDate = (d: string | null | undefined) => formatDate(d, i18n.language);
   // Track the playing item by id (not a frozen index) so it survives the queue changing
   // under us — e.g. an item removed in another tab. The index is derived from the *live*
   // queue, so the N / M counter and prev/next neighbours stay correct without disrupting
@@ -235,8 +228,14 @@ export default function PlayerModal({
   // Auto-advance + loop are persistent per-user settings. Read the current values from the cached
   // `me`, mirror them in local state for instant UI, and write both to the server + cache on change
   // so they apply to every player and survive reloads. A ref feeds the (once-bound) end handler.
-  const savedPrefs = (qc.getQueryData<{ preferences?: Record<string, unknown> }>(["me"])
-    ?.preferences ?? {}) as { playerAutoAdvance?: AutoMode; playerLoop?: LoopMode };
+  const savedPrefs = useMemo(
+    () =>
+      (qc.getQueryData<{ preferences?: Record<string, unknown> }>(["me"])?.preferences ?? {}) as {
+        playerAutoAdvance?: AutoMode;
+        playerLoop?: LoopMode;
+      },
+    [qc]
+  );
   const [autoMode, setAutoMode] = useState<AutoMode>(savedPrefs.playerAutoAdvance ?? "off");
   const [loopMode, setLoopMode] = useState<LoopMode>(savedPrefs.playerLoop ?? "off");
   const modeRef = useRef({ autoMode, loopMode });
@@ -446,7 +445,10 @@ export default function PlayerModal({
     // navigated to via description links.
     const maybeAutoWatch = (current: number, duration: number): boolean => {
       if (autoMarkedRef.current || duration <= 0 || currentIdRef.current !== id) return false;
-      if (current > duration - FINISH_MARGIN) {
+      // Cap the finish margin at half the clip, so a video shorter than FINISH_MARGIN isn't
+      // auto-marked watched almost immediately (its threshold would otherwise go negative).
+      const margin = Math.min(FINISH_MARGIN, duration / 2);
+      if (current > duration - margin) {
         autoMarkedRef.current = true;
         setWatched(true);
         return true;
@@ -646,7 +648,7 @@ export default function PlayerModal({
         {hasQueue && (
           <div className="flex flex-wrap items-center justify-center gap-x-4 gap-y-1.5 px-4 py-2 border-b border-border text-sm">
             <button
-              onClick={() => index > 0 && setPlayingId(queue![index - 1].id)}
+              onClick={goPrev}
               disabled={index === 0}
               title={`${t("player.previous")} · Shift+←`}
               className="inline-flex items-center gap-1.5 text-muted enabled:hover:text-fg disabled:opacity-40 transition"
@@ -657,7 +659,7 @@ export default function PlayerModal({
               {index + 1} / {queue!.length}
             </span>
             <button
-              onClick={() => index < queue!.length - 1 && setPlayingId(queue![index + 1].id)}
+              onClick={goNext}
               disabled={index === queue!.length - 1}
               title={`${t("player.next")} · Shift+→`}
               className="inline-flex items-center gap-1.5 text-muted enabled:hover:text-fg disabled:opacity-40 transition"

frontend/src/components/VideoCard.tsx

@@ -14,7 +14,7 @@ import AddToPlaylist from "./AddToPlaylist";
 import DownloadButton from "./DownloadButton";
 import clsx from "clsx";
 import type { Video } from "../lib/api";
-import { formatDuration, formatViews, relativeTime } from "../lib/format";
+import { formatDate, formatDuration, formatViews, relativeTime } from "../lib/format";
 
 function Actions({
   video,
@@ -257,16 +257,7 @@ function VideoCard({
         </>
       )}
       {relativeTime(video.published_at)}
-      {video.published_at && (
-        <>
-          {" · "}
-          {new Date(video.published_at).toLocaleDateString(i18n.language, {
-            year: "numeric",
-            month: "short",
-            day: "numeric",
-          })}
-        </>
-      )}
+      {video.published_at && <>{" · "}{formatDate(video.published_at, i18n.language)}</>}
     </>
   );
   // Show the full title in a native tooltip only when it's clamped (overflows its 2 lines).