Optional transparent / acrylic main window

Problem

Right now the Fluxer desktop client does not support a transparent main window. I can set all background theme tokens (e.g. --background-primary, --background-secondary, etc.) to rgba(0,0,0,0) using the custom CSS overrides, which makes the UI backgrounds transparent. However, the actual app window itself is still opaque, so I only see a solid window background instead of my desktop/compositor behind Fluxer. This makes it impossible to integrate Fluxer cleanly with my transparent desktop setup (Hyprland style transparency).

Proposed solution

Add an option for a transparent (or acrylic/blurred) main window in the desktop client.

• Enable a transparent window mode in the Electron configuration (e.g. transparent: true on the BrowserWindow and no solid background color).
• (Optional) Support platform‑specific blur/acrylic effects where available.

With this in place, users who already set --background-* tokens to rgba(0,0,0,0) can have the Fluxer UI and window fully transparent down to the desktop.