[Linux] Window top corners are not rounded

## Description

On Linux, the top two corners of both the launcher window and spawned ComfyUI instance windows are not rounded, making them look inconsistent compared to the rest of the desktop environment (especially on GNOME/KDE with rounded-corner themes).

## Root Cause

This is a known Electron limitation ([electron/electron#33036](https://github.com/electron/electron/issues/33036)). The `roundedCorners` BrowserWindow option only works on **macOS and Windows**. On Linux, Electron/Chromium does not natively apply rounded corners to windows. The `titleBarStyle: 'hidden'` + `titleBarOverlay` configuration used in `src/main/index.ts` produces square top corners on Linux.

**Affected windows:**
- Main launcher window (`createMainWindow` — line ~251)
- ComfyUI instance windows (`comfyWindow` — line ~630)

## Possible Fix

The standard workaround is to use `transparent: true` + `frame: false` on Linux only, then apply `border-radius` + `overflow: hidden` via CSS on the root content element, with custom window control buttons rendered in HTML. This is the approach used by VS Code and other Electron apps.

**Trade-offs:**
- Adds Linux-specific code paths and custom window controls
- `transparent: true` can have performance implications on some Linux compositors
- Resizing behavior may differ without native frame

## Priority

Low — Linux is lower priority than macOS/Windows. Documenting for future reference.