Desktop: Fix Responsive Menu

[dbpolito]

The data-tauri-drag-region attribute was on the header which contains interactive elements like the mobile menu button. In Tauri, this attribute makes the element act as a window drag area, which intercepts click events before they reach child elements.

Since there's already a dedicated drag region div in packages/desktop/src/index.tsx:201 for macOS, the header doesn't need this attribute. Removing it allows click events to properly propagate to the mobile menu button.