fix: make build scripts cross-platform with auto platform detection

[fancyboi999]

Summary

• replace Windows-only PowerShell prebuild logic with a cross-platform Node cleanup script
• add platform-aware build routing so npm run build auto-selects build:win / build:mac / build:linux
• split build pipeline into reusable steps (build:app, pack:*) and add explicit scripts for each OS
• update electron-builder config so macOS/Linux use build/icon.png and add mac/linux targets

Why

Issue #1 reports npm run build fails on macOS/Linux because prebuild hardcodes powershell.

Verification

Executed on macOS:

• npm run build -> auto-routed to build:mac, succeeded (produced .dmg and .zip)
• npm run build:linux -> succeeded (produced .AppImage)
• npm run build:win -> succeeded (produced setup.exe)

Closes #1