fix(ui): a picture that fails to load shows the card's own frame, then retries

A bare <img> sent a Referer that Google's image CDNs refuse and had no retry and no fallback, so a
Home shelf card showed the browser's broken-image mark. ui/RemoteImage gathers what Avatar, the
channel banner and the download rows had each hand-rolled: no Referer, up to three retries past the
browser's negative cache, the caller's fallback (by default the card's empty frame) instead of the
broken mark, the failure remembered by URL. Every picture now goes through it; a lint rule refuses
a bare <img>.

frontend/src/components/VideoCard.tsx

@@ -43,6 +43,7 @@ import {
 import { MediaCard, MediaProgress } from "./ui/MediaCard";
 import { MediaResumeButtons } from "./MediaResumeButtons";
 import { RelativeTime } from "./ui/RelativeTime";
+import RemoteImage from "./ui/RemoteImage";
 
 /** The "…" that opens the video's action list, and whether its menu is the open one. */
 type More = { props: Record<string, unknown>; open: boolean };
@@ -252,7 +253,7 @@ function Thumb({
         className={MEDIA_PICTURE}
       >
         {video.thumbnail_url ? (
-          <img
+          <RemoteImage
             src={video.thumbnail_url}
             alt=""
             loading="lazy"