fix(channel): no broken-image glyph while the banner retries (E28)

The banner is retried on error; each failed attempt showed the browser's
broken-image glyph for as long as the retry waited. The <img> now stays
invisible until its source has actually loaded.

frontend/src/components/ChannelPage.tsx

@@ -88,6 +88,9 @@ export default function ChannelPage({
   // only fall back to the blank bar if it keeps failing (a genuinely dead URL — rare).
   const MAX_BANNER_RETRIES = 3;
   const [bannerAttempt, setBannerAttempt] = useState(0);
+  // The banner source that has actually loaded. Until then the <img> is invisible: an attempt that
+  // fails shows the browser's broken-image glyph for as long as the retry waits (seen live, E28).
+  const [bannerLoaded, setBannerLoaded] = useState<string | null>(null);
 
   // A LIVE query: while THIS user's full-history backfill is running, poll so the stored count (and
   // the N/S/L breakdown) climb on their own — the immediate backfill fills it in seconds, and the
@@ -319,12 +322,15 @@ export default function ChannelPage({
   // The channel page owns no scroller of its own (PageShell provides the one app scroller). Its
   // whole chrome is fixed — wrapped in <PageToolbar> it portals into the shell's fixed band, so the
   // banner/identity/tabs/toolbar stay put and only the video grid scrolls.
+  const bannerSrc = ch?.banner_url
+    ? `${ch.banner_url}=w1707${bannerAttempt ? `?r=${bannerAttempt}` : ""}`
+    : null;
   return (
     <>
       <PageToolbar>
         {/* Banner + back — OPTION C: inset, rounded card (not full-bleed) */}
         <div className="relative px-4 sm:px-6 pt-3">
-          {ch?.banner_url && bannerAttempt <= MAX_BANNER_RETRIES ? (
+          {bannerSrc && bannerAttempt <= MAX_BANNER_RETRIES ? (
             // Match YouTube's banner: the stored bannerExternalUrl is the full 16:9 template at a
             // low default size, so request a crisp wide version (=w1707) and object-cover the
             // desktop "safe area" — the centre 2560×423 (~6:1) band.
@@ -336,9 +342,12 @@ export default function ChannelPage({
                 // key forces a fresh <img> per attempt; the ?r= cache-buster on a retry bypasses the
                 // browser's negative cache (googleusercontent accepts an extra query param).
                 key={bannerAttempt}
-                src={`${ch.banner_url}=w1707${bannerAttempt ? `?r=${bannerAttempt}` : ""}`}
+                src={bannerSrc}
                 alt=""
-                className="w-full h-full object-cover object-center"
+                className={`w-full h-full object-cover object-center ${
+                  bannerLoaded === bannerSrc ? "" : "opacity-0"
+                }`}
+                onLoad={() => setBannerLoaded(bannerSrc)}
                 onError={() => {
                   if (bannerAttempt <= MAX_BANNER_RETRIES) {
                     window.setTimeout(