Refine floating shelf presentation

[brzvsk]

Summary

• remove the extra glass gutter and clip the rendered shelf cleanly to its rounded shape
• replace fade-heavy transitions with fast bottom-edge slide animations and keep the previous app frontmost
• remove the header separator, tighten card spacing, and make the pinboard hit-test backing visually neutral
• make emoji the only optional pinboard identity by removing tint dots, tinted tab backgrounds, and color controls
• replace the dark-shelf toggle with Follow System, Light, and Dark themes that also apply to Settings
• migrate the previous dark-shelf preference without changing existing users’ appearance

Verification

• swift test --package-path CopyCore — 159 tests passed
• signed Debug archive built successfully
• manually verified shelf corners, animation, focus retention, theme switching, Settings appearance, and spacing on macOS