Closes #189.

Add an invisible, full-width 8px drag strip in the existing top gutter for native layouts. It sits outside panel no-drag regions and above the full-height sidebar, without moving content or covering tabs below the gutter. Browser layouts are unchanged; the strip is hidden from accessibility navigation.

Validation: `bun run ci`, `bun types`, production Vite build, and signed Ace-dev build pass. Native screenshot confirms unchanged dashboard layout. Actual pointer-drag verification remains manual: desktop_focus refused with a timeout; one background drag was accepted but window geometry stayed unchanged (existing #106/#68 tool limitations). No successful drag or tab/control regression check is claimed. Owned dev app and host stopped.

### Built in Ace

Entirely in Ace using a lane and shell/file tools; native checks used Ace desktop tools. No external editor, terminal, agent, or input fallback.