improvement(plex): serve right-sized poster/art images via Plex transcode

Posters/art were proxied at full resolution (many 1-7MB, some 9.5MB) into ≤176px
grid/info cells — heavy to fetch + decode, so fast scrolling lagged 1-2s even
with a warm cache (the cache removed the Plex round-trip, not the image weight).

Now PlexClient.image_bytes optionally hits Plex's /photo/:/transcode to resize
server-side; the /image endpoint requests thumb=400x600, art=1280x720 and keys
the cache by width (old full-size files orphaned, re-cached small). Measured:
a poster 5.5MB->50KB, art 8.1MB->137KB, at 400x600 (crisp on 2x DPR). Grid
scroll is near-instant and the disk cache shrinks ~8x.

frontend/src/lib/releaseNotes.ts

@@ -14,6 +14,14 @@ export interface ReleaseEntry {
 }
 
 export const RELEASE_NOTES: ReleaseEntry[] = [
+  {
+    version: "0.28.2",
+    date: "2026-07-06",
+    summary: "Much faster, smoother Plex browsing — right-sized poster images.",
+    features: [
+      "Plex images: posters and art are now resized by Plex to the size actually shown (instead of shipping the full-resolution originals — some were several MB each), so the movie grid scrolls smoothly and images pop in almost instantly. Also shrinks the image cache dramatically.",
+    ],
+  },
   {
     version: "0.28.1",
     date: "2026-07-06",