Fix gallery performance by downsampling thumbnails

[shawonashraf]

Summary

• replace full-image decoding in ImageCardView with ImageIO thumbnail downsampling
• add an in-memory thumbnail cache keyed by file path and target thumbnail size
• pass gallery zoom-driven pixel target into each card so thumbnails scale with UI density
• keep ImageCardView source-compatible via an explicit initializer with a default thumbnail size so both 2-arg and 3-arg call sites compile
• guard async thumbnail tasks from applying stale results after zoom changes
• break large GalleryView image-card/context-menu expression into smaller helpers to resolve Swift type-checker timeout

Testing

• xcodebuild -project Wardrobe.xcodeproj -scheme Wardrobe build (blocked in cloud Linux environment: xcodebuild: command not found)

Fixes #7