2025-10-07: Add reusable backdrop utility and enhance scooter terminal

[john-mutuma]

What does this PR do?

Adds a reusable backdrop utility function and integrates it with the scooter terminal for better visual depth and code reusability.

Changes:

• Extract create_backdrop function in windows.lua for creating backdrop windows
• Add automatic highlight setup to create_backdrop function
• Refactor with_win_backdrop to use the new create_backdrop function (eliminating ~35 lines of duplicate code)
• Add backdrop support to scooter terminal with on_open/on_close lifecycle management
• Update scooter terminal configuration (winblend: 12, width: 175)
• Adjust lazygit window scaling factor from 0.85 to 0.8
• Update scooter.config.toml preview settings

Why is it needed?

1. Code Reusability - The backdrop creation logic was duplicated between functions; now it's a reusable utility that can be used across the codebase
2. Better UX - Semi-transparent backdrop (60% blend) provides visual separation for floating windows, making the scooter terminal stand out
3. Automatic Management - Backdrop lifecycle is tied to window lifecycle via callbacks, ensuring no manual cleanup is required
4. Cleaner API - Highlight setup is now handled automatically by create_backdrop, eliminating duplicate setup code
5. Proper Z-indexing - Backdrop uses z-index 39, positioned below terminal's default 40, ensuring correct visual layering

How have the changes been tested?

• Code compiles without errors
• No LSP diagnostics in modified files
• Refactoring maintains existing functionality of with_win_backdrop
• Manual testing pending: Open scooter terminal and verify backdrop appears/disappears correctly

Screenshots (if applicable)

Screenshots will be added after manual testing of the backdrop behavior.

Checklist

• Code follows project Lua conventions and structure
• Functions are properly documented with luadoc comments
• No duplicate code - eliminated ~35 lines of duplication
• Changes committed with descriptive message following conventional commits format
• All diagnostics resolved
• Manual testing completed (scooter terminal open/close with backdrop)
• Verified no z-index conflicts with other floating windows
• Tested visual selection search functionality