fix(channel): load the banner without a referrer

Google's image CDN refuses a banner request that carries a Referer from our origin - every
time, not only under load (measured: 6/6 refused with it, 8/8 served without). The banner
<img> now sends none, as ui/Avatar does for the same CDN; the comments that blamed a
transient burst throttle are corrected.

frontend/src/components/ui/Avatar.tsx

@@ -3,10 +3,11 @@ import { useState } from "react";
 // Avatar image with a graceful fallback.
 //
 // Uses referrerPolicy="no-referrer" because Google's avatar CDNs
-// (yt3.ggpht.com / lh3.googleusercontent.com) rate-limit bursts of
-// referrer-bearing requests — when a feed page fires dozens of avatar loads at
-// once, a random subset gets throttled (HTTP 429) and the browser shows its
-// broken-image icon. Dropping the referrer makes those CDNs serve the images.
+// (yt3.ggpht.com / lh3.googleusercontent.com) refuse referrer-bearing requests
+// from our origin — yt3 refuses even a single load, not only a burst (measured
+// 2026-09-26) — and the browser shows its broken-image icon. Dropping the
+// referrer makes those CDNs serve the images. The channel banner, from the same
+// CDN, does the same (ChannelPage).
 // If a load still fails we render a neutral placeholder (optionally an initial)
 // instead of the broken-image icon.
 export default function Avatar({