perf(media): request display-sized cover thumbnails #285

There is no single optimal source-art size: rendered cover dimensions vary by display resolution, grid shape, list/detail layout, and device pixel density. Users should retain good-quality source artwork rather than manually pre-scaling every image.

Implemented pipeline

• Frontend calculates cover requirements from actual grid and detail viewport geometry.
• Grid and detail views request independent image sizes.
• Frontend sends bounded media.image requests through maxSize.
• Core snaps requests to bounded thumbnail tiers: 32, 64, 128, 256, 512, and 768.
• Core resizes once and caches lossy WebP thumbnails.
• MiSTer can consume Core-owned local thumbnail paths instead of transferring base64 payloads.
• Frontend caches decoded images in process memory with a strict 128 MiB LRU cap.

This keeps transferred and decoded images close to their rendered size without sacrificing detail-pane quality or requiring users to maintain device-specific artwork copies.

Implemented by ZaparooProject/zaparoo-core#1034 and frontend #377.