Add frame preset swatch selector

[yvette-carlisle]

Summary

• Replace Frame Preset dropdown with compact background swatches and off slash swatch
• Add mouse-accessible chevron stepping with click-and-hold repeat for long preset lists
• Keep wallpaper preview decoding/cache Rust-owned and avoid synchronous full-wallpaper Swift decoding

Verification

• cargo make check-swift
• cargo make check-rust
• cargo make fmt-check
• cargo make check-vstyle
• cargo test -p rsnap-host-ffi ffi_capture_frame_wallpaper_png_thumbnail_returns_owned_region
• semantic_drift_audit.py --repo . --rev origin/main
• git diff --check HEAD~1..HEAD