[BUG] Desktop v2 beta: window is undraggable with "Tabs: Vertical" (only the sidebar clearance strip works)

Description

On the desktop v2 beta with Tabs: Vertical, the only draggable area in the window is the clearance strip at the top of the vertical-tabs sidebar (next to the macOS traffic lights). The rest of the top strip looks empty but is undraggable — grabbing it does not move the window.

With Tabs: Horizontal the draggable sliver at the top is at least reachable; with Vertical it effectively disappears.

Environment

• OpenCode Desktop (Beta) 0.0.0-beta-19425 (ai.opencode.desktop.beta)
• macOS 27.0 (build 26A428)

Steps to reproduce

1. Open the v2 beta desktop app with the new layout.
2. Set Tabs: Vertical.
3. Try to move the window by dragging the empty strip along the top edge → nothing happens.
4. Drag the top of the left sidebar (around the traffic lights) → the window moves.

Expected

The empty top strip should remain a drag region in the vertical tabs layout, with interactive controls marked no-drag (same as the horizontal tabs layout).

Additional context

• packages/ui/src/styles/base.css still only applies unprefixed app-region: drag to [data-tauri-drag-region], without a -webkit-app-region fallback.
• Related (v1 desktop): #41646, and #34606 (macOS titlebar fix, closed unmerged).