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.

VERSION

@@ -1 +1 @@
-0.28.1
+0.28.2