Real-image posters of library titles: use the 500 px copy

Library titles fetched Radarr's/Sonarr's 250×375 poster copy, which
real images show at up to 640 pixels, so movie posters looked soft next
to series posters that came from the provider's full-size original
(Sonarr behind a login). With real images the 500×750 copy is used;
block-character posters keep the small one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- **Sharper library posters as real images:** titles in the library used Radarr's/Sonarr's
+  250×375 copy of the poster, which real images (kitty, Ghostty) enlarge to up to 640 pixels and
+  so looked soft. Real images now use the 500×750 copy; block-character posters keep the small one.
 - **First start without a config file:** arr-tui creates the file (private, with its directory)
   and opens the Config view to enter the services, instead of refusing to start. Services may be
   left out: one without address and key is not used, its indicator stays grey and its lists say