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/lib/plexImage.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+
+import { cardImage } from "./plexImage";
+
+describe("cardImage", () => {
+  it("offers a poster's small cut beside the full one", () => {
+    expect(cardImage("/api/plex/image/123")).toEqual({
+      src: "/api/plex/image/123",
+      srcSet: "/api/plex/image/123?variant=thumb_sm 240w, /api/plex/image/123 400w",
+      sizes: "auto, 200px",
+    });
+  });
+
+  it("offers a backdrop's small cut beside the full one", () => {
+    expect(cardImage("/api/plex/image/123?variant=art").srcSet).toBe(
+      "/api/plex/image/123?variant=art_sm 480w, /api/plex/image/123?variant=art 1280w",
+    );
+  });
+
+  it("leaves any other picture alone", () => {
+    expect(cardImage("https://image.tmdb.org/x.jpg")).toEqual({
+      src: "https://image.tmdb.org/x.jpg",
+    });
+    expect(cardImage("")).toEqual({ src: "" });
+  });
+});