Posters: always a 2:3 box

The poster box followed the image's shape, so images Radarr/Sonarr hand
out in another shape came out smaller than the others in a list. The box
is now always 2:3: an image within 12% of that fills it, trimmed at the
long sides (block characters too, which were stretched before); one
further off is shown whole, centred in the box.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+- **Posters all the same size:** the details pane always shows a poster in a 2:3 box. Images
+  slightly off that shape (some Radarr/Sonarr hand out) fill it, trimmed at the long sides; images
+  far from it (square, landscape) are shown whole and centred. Before, such posters came out
+  smaller than the others in the same list.
 - **Posters and casts are kept between runs** (in `~/.cache/arr-tui`; posters 30 days and at
   most 200 MB, casts 7 days), so they show at once after a restart and are not downloaded again.
 - **Downloading titles in the Library:** the dot before a movie or series turns blue while it is in