[codex] Fix Windows build copy step

[fusae]

What changed

• replaced the Unix-only copy-files npm script with a Node-based cross-platform script
• added scripts/copy-files.mjs to copy db, data, and web assets during build

Why

• the documented Windows build path fails in native Windows shells because mkdir -p and cp are not available
• this makes Build-Windows.cmd and npm run desktop:dist:win work as documented

Validation

• npm run build
• npm run desktop:dist:win on Windows with Node 20