fix(ui): robust avatars — no-referrer + graceful fallback

Channel/account avatars come from Google's image CDNs (yt3.ggpht.com,
lh3.googleusercontent.com). On a feed page dozens load at once; the CDN
rate-limits the referrer-bearing burst (429), so a random subset rendered the
browser's broken-image icon (the URLs themselves are valid — verified 200).

Add a shared <Avatar> that sets referrerPolicy="no-referrer" (which the CDNs
serve without throttling) and falls back to a neutral initial placeholder on
error instead of the broken-image icon. Use it for video-card, player, channel
manager, header and settings avatars.

frontend/src/components/PlayerModal.tsx

@@ -2,6 +2,7 @@ import { useEffect, useRef, useState, type ReactNode } from "react";
 import { createPortal } from "react-dom";
 import { useQuery } from "@tanstack/react-query";
 import { ArrowLeft, Check, CheckCheck, X } from "lucide-react";
+import Avatar from "./Avatar";
 import { api, type Video } from "../lib/api";
 import { formatDuration, formatViews, relativeTime } from "../lib/format";
 
@@ -454,10 +455,10 @@ export default function PlayerModal({
               When navigated to a linked video we only have its author (from the player),
               so we show that as plain text and hide feed-video-specific bits. */}
           <div className="flex items-center gap-3 mt-3">
-            {!navigated && video.channel_thumbnail && (
-              <img
+            {!navigated && (
+              <Avatar
                 src={video.channel_thumbnail}
-                alt=""
+                fallback={video.channel_title ?? ""}
                 className="w-9 h-9 rounded-full shrink-0"
               />
             )}