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/PosterTile.tsx

@@ -3,6 +3,7 @@ import { hueOf, hueTileGradient } from "../lib/hueTile";
 import { navItemAttrs } from "../lib/listNav";
 import { MEDIA_IMG } from "./ui/mediaCardStyles";
 import { Pressable } from "./ui/Pressable";
+import { cardImage } from "../lib/plexImage";
 
 // A browse tile split in two: the TOP half shows up to 2 montage posters (from /api/plex/image/{key}),
 // the BOTTOM half is a solid colour-hash band that carries the label — so the text is always readable
@@ -50,7 +51,7 @@ export default function PosterTile({
             posters[i] ? (
               <img
                 key={i}
-                src={`/api/plex/image/${posters[i]}`}
+                {...cardImage(`/api/plex/image/${posters[i]}`)}
                 alt=""
                 loading="lazy"
                 className={MEDIA_IMG}