Replace the `github.com/strukturag/libheif-go` CGo dependency with
macOS-native `sips` for HEIC/AVIF wallpaper thumbnail conversion.
- Drop `libheif` from go.mod, Makefile, Goreleaser config, and CI
- Remove `cgo` build tag requirement from wallpaper package
- Implement `convertWithSips()` using temp files and the `sips` CLI
- Refactor thumbnail decoding into `extractThumbnailPreview()` helper
- Update tests to cover sips-based HEIC conversion path
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