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/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).