fix: force macOS wallpaper refreshes with versioned copies

[TyceHerrman]

Summary

Work around macOS wallpaper caching by giving each successful update a unique file path instead of repeatedly pointing System Events at satpaper_latest.png.

Changes

• copy the generated wallpaper to a uniquely named hidden sibling before applying it
• pass the wallpaper path to osascript as an argument instead of interpolating it into AppleScript
• update every desktop and report non-zero osascript status with stderr context
• retain the new copy only after a successful switch, then remove the superseded copy
• perform bounded startup cleanup of stale Satpaper-generated copies after the first successful switch
• refuse to remove files that do not match Satpaper's reserved filename format
• handle filename collisions with exclusive file creation
• add tests for generated names, cleanup limits, and stale-copy removal safety

Testing

• cargo test --release -- --skip generate_wallpaper — 4 passed
• cargo clippy --all-targets --all-features -- -D warnings
• git diff --check
• manual end-to-end wallpaper refresh on macOS Sequoia (not run because it would change the active desktop)

Closes #30