perf(plex): a card fetches its picture at a card's size, not the detail page's

Cards fetched the full cuts: a 400x600 poster (~38 KB) for a ~144px picture and the 1280x720
backdrop (~162 KB) for a ~212px shelf picture. The image proxy now also cuts thumb_sm (240x360)
and art_sm (480x270), cached apart; card pictures (grid, shelves, collection tiles, Up next) carry
both in srcset with sizes=auto, so the browser picks by laid-out width and DPR. Localdev:
17 KB a poster, 56 KB a backdrop. The detail page keeps the full cuts.

frontend/src/components/UpNextCard.tsx

@@ -5,6 +5,7 @@ import { Play, X } from "lucide-react";
 import { api } from "../lib/api";
 import { plexLang } from "../i18n";
 import { qk } from "../lib/queryKeys";
+import { cardImage } from "../lib/plexImage";
 import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
 
@@ -72,8 +73,9 @@ export function UpNextCard({
       <div className="h-[6.5rem] w-[4.6rem] flex-none overflow-hidden rounded-lg bg-white/10">
         {next && (
           <img
-            src={api.plexImageUrl(nextId, "thumb")}
+            {...cardImage(api.plexImageUrl(nextId, "thumb"))}
             alt=""
+            loading="lazy"
             className="h-full w-full object-cover"
             draggable={false}
           />