fix(images): scale artwork budgets to the physical display

Every artwork budget in the image pipeline was tuned for 1080p surfaces:
the transcode request clamp (1920x1080), the per-type decode caps
(poster 720x1080, thumb 960x540, heroLogo 1000x500, ...) and the TV
image-cache bytes. Those numbers are exact on phones and on the many TV
boxes that composite the app at 1080p, but a TV compositing at 4K
renders every capped image below its slot and GPU-upscales the result:
hero backdrops by 2x, hero logos by ~1.8x, wide episode thumbs by ~1.3x,
shelf posters by ~1.13x - the softness reported against the official
Plex client in #1697, and the class #860's min-2x-DPR fix could not
reach.

DevicePerformance now latches a display budget factor - the display's
shortest physical axis over 1080, capped at 2x - whenever the image
cache budget is applied (startup, post-mount, effects-setting changes).
The transcode clamp, the full-tier decode caps and the TV cache bytes
all scale by it, so a 4K surface fetches and decodes 4K backdrops and
proportionally larger cards. The reduced tier stays pinned to 1.0, and
sub-2.5GiB hardware holds the factor at 1.5 so full-budget 4K art
(~33MB per decode) cannot starve mid-RAM boxes; latching once per
session keeps transcode URLs - and with them the disk cache keys -
stable across rotation and rebuilds.

Whether a given TV composites at 1080p or 4K decides whether any of
this can help, and logs never recorded it: the startup banner and the
log-upload header now carry a display line (physical, logical, DPR,
latched budget) so uploaded logs answer that question directly.

The two pre-existing Windows-host test failures (automotive auto-PiP
gate, backdrop temp-dir teardown lock) reproduce unchanged on the base
commit.

lib/screens/settings/logs_screen.dart

@@ -127,6 +127,7 @@ class _LogsScreenState extends State<LogsScreen> with MountedSetStateMixin {
     }
 
     buffer.writeln('Effects: ${DevicePerformance.describeSync()}');
+    buffer.writeln('Display: ${DevicePerformance.describeDisplay()}');
 
     setStateIfMounted(() => _deviceInfo = buffer.toString().trimRight());
   }