Add drag-to-snap areas for eighths

[devindudeman]

Summary

• Adds compound snap area support for eighths (4x2 grid), following the same pattern used by sixths and fourths
• Dragging to the top or bottom screen edge and sliding along it snaps windows to one of four eighth positions
• Selectable in the Snap Areas preferences tab dropdown
• Eighths appear in their own "Eighths" submenu in the status bar menu
• Particularly useful for ultra-wide monitors

Changes

• New EighthsCompoundCalculation.swift with top/bottom eighths compound calculations
• CompoundSnapArea.swift — added topEighths/bottomEighths cases
• WindowAction.swift — made eighths drag-snappable, added display names, category, and firstInGroup
• WindowActionCategory.swift — added .eighths category
• TerminalCommands.md — updated docs

Test plan

• In Settings > Snap Areas, set the top edge dropdown to "Top eighths from corners; maximize"
• In Settings > Snap Areas, set the bottom edge dropdown to "Bottom eighths from corners; thirds"
• Drag window to top edge, slide left/right — should cycle through 4 top eighth positions
• Drag window to bottom edge, slide left/right — should cycle through 4 bottom eighth positions
• Verify eighths appear in "Eighths" submenu in status bar menu
• Verify existing sixths/thirds/fourths snap areas still work correctly

[rxhanson]

Thanks! I'll test it out soon