Add optional transparent Linux desktop windows

[wy3z]

Human comments

Small fix to allow transparency - added as an option. Maybe opinionated but suggest it's a default as can't see it causing any issues.

What was wrong

Electron creates opaque desktop windows by default, so Linux compositors cannot show transparency or blur behind bb even when a custom CSS theme uses translucent colors. This leaves compositor-level effects unavailable after the frameless Linux window option added in #1785. Fixes #1916.

What changed

• Added a Linux-only --transparent-window startup flag.
• Set Electron's transparent: true and backgroundColor: "#00000000" window options when requested.
• Kept the option independent from --no-window-frame; both remain opt-in and can be combined.
• Added argument parsing and BrowserWindow option coverage.
• Documented Linux usage, restart requirements, and Electron transparency limitations.
• No host-daemon protocol changes.

How you verified

• pnpm exec turbo run typecheck --filter=@bb/desktop
• pnpm exec turbo run test --filter=@bb/desktop --force (37 files, 243 tests passed)
• pnpm exec prettier --check apps/desktop/README.md apps/desktop/src/desktop-window-factory.ts apps/desktop/src/desktop-window-transparency.ts apps/desktop/src/main.ts apps/desktop/test/desktop-window-factory.test.ts apps/desktop/test/desktop-window-transparency.test.ts
• git diff --check

Fixes #1916

AGENT GENERATED