feat(desktop): hover X close button on zone tabs

Add a hover-to-close X button to PaneTab (the fancy-zones tab shell).
The button slot is always reserved inline (shrink-0) so the tab width
stays stable whether the X is visible or not — no layout shift on hover.
Visible on group-hover/tab, hidden by default via opacity transition.

The dirty dot yields to the X when both are present (closeable + dirty):
the X wins on hover, the dot shows otherwise. Vertical tabs skip the X
(writing-mode:vertical-rl makes an inline button awkward) and keep the
absolute-positioned dirty dot.

Pointerdown on the X is stopped so the tab's drag/activate handlers
never fire — the X is a leaf close action, not a drag start.

apps/desktop/src/components/ui/pane-tab.test.tsx

@@ -90,3 +90,54 @@ describe('PaneTab close gestures', () => {
     expect(onPointerDown).toHaveBeenCalledTimes(1)
   })
 })
+
+describe('PaneTab hover close button', () => {
+  it('renders a close button when onClose is set on a horizontal tab', () => {
+    const onClose = vi.fn()
+    render(
+      <PaneTab onClose={onClose}>
+        <PaneTabLabel>tab</PaneTabLabel>
+      </PaneTab>
+    )
+
+    expect(screen.getByRole('button', { name: 'Close tab' })).toBeTruthy()
+  })
+
+  it('clicking the close button calls onClose and stops propagation', () => {
+    const onClose = vi.fn()
+    const onTabPointerDown = vi.fn()
+    render(
+      <PaneTab onClose={onClose} onPointerDown={onTabPointerDown}>
+        <PaneTabLabel>tab</PaneTabLabel>
+      </PaneTab>
+    )
+
+    const closeBtn = screen.getByRole('button', { name: 'Close tab' })
+    fireEvent.pointerDown(closeBtn)
+    fireEvent.click(closeBtn)
+    expect(onClose).toHaveBeenCalledTimes(1)
+    // The tab's own pointerdown handler must NOT fire — the X is a leaf action.
+    expect(onTabPointerDown).not.toHaveBeenCalled()
+  })
+
+  it('does not render a close button on vertical tabs', () => {
+    const onClose = vi.fn()
+    render(
+      <PaneTab onClose={onClose} vertical>
+        <PaneTabLabel>tab</PaneTabLabel>
+      </PaneTab>
+    )
+
+    expect(screen.queryByRole('button', { name: 'Close tab' })).toBeNull()
+  })
+
+  it('does not render a close button without onClose', () => {
+    render(
+      <PaneTab>
+        <PaneTabLabel>tab</PaneTabLabel>
+      </PaneTab>
+    )
+
+    expect(screen.queryByRole('button', { name: 'Close tab' })).toBeNull()
+  })
+})

apps/desktop/src/components/ui/pane-tab.tsx

@@ -1,5 +1,6 @@
 import * as React from 'react'
 
+import { Codicon } from '@/components/ui/codicon'
 import { cn } from '@/lib/utils'
 
 /** Inset bottom stroke for a horizontal tab strip — titlebar color, cut by the active tab. */
@@ -27,8 +28,8 @@ const TAB_IDLE =
 interface PaneTabProps extends React.ComponentProps<'div'> {
   active?: boolean
   dirty?: boolean
-  /** Close gesture, no hover X (too easy to hit on small tabs): middle-click,
-   *  or ⌘-click as the trackpad-friendly Mac equivalent. */
+  /** Close gesture: hover X on horizontal tabs, plus middle-click and ⌘-click
+   *  (the trackpad-friendly Mac equivalent). */
   onClose?: () => void
   /** Vertical rail form (collapsed sidebar zones). */
   vertical?: boolean
@@ -127,7 +128,35 @@ export const PaneTab = React.forwardRef<HTMLDivElement, PaneTabProps>(function P
       {...props}
     >
       {children}
-      {dirty && (
+      {onClose && !vertical && (
+        <button
+          aria-label="Close tab"
+          className={cn(
+            'grid size-4 shrink-0 place-items-center self-center rounded-sm text-(--ui-text-tertiary) transition-opacity',
+            // Always reserve the slot; visible on hover. The dirty dot
+            // (below) yields to the X on hover so the two never overlap.
+            // Opacity transitions keep the layout stable (no width shift
+            // when the X appears).
+            'mr-1.5 opacity-0 group-hover/tab:opacity-100',
+            'hover:bg-(--ui-control-hover-background) hover:text-foreground'
+          )}
+          onClick={event => {
+            event.preventDefault()
+            event.stopPropagation()
+            onClose()
+          }}
+          onPointerDown={event => {
+            // Claim the press so the tab's drag/activate pointerdown handler
+            // can't fire — the X is a leaf action, never a drag start.
+            event.preventDefault()
+            event.stopPropagation()
+          }}
+          type="button"
+        >
+          <Codicon name="close" size="0.625rem" />
+        </button>
+      )}
+      {dirty && !(onClose && !vertical) && (
         <span
           aria-hidden
           className={cn(