fix: Mac window becomes unmovable after pane rearrangement (#10133)

On main, after rearranging panes, the window becomes permanently
immovable. Grab handles temporarily set `window.isMovable = false` on
hover to prevent window dragging from interfering with pane drags
(fixing [#10110](https://github.com/ghostty-org/ghostty/issues/10110)),
but the restoration logic failed when views were destroyed during pane
rearrangement (which happens each time a pane is rearranged).

The previous approach managed `window.isMovable` state across the view
lifecycle:

1. `mouseEntered` → saved and disabled `window.isMovable`
2. View removed during rearrangement → `mouseExited` never fired
3. `deinit` ran with `self.window` already nil → restoration failed
4. Window stuck with `isMovable = false`

Instead of managing window state, prevent the mouseDown event from
reaching the window's drag handler by overriding mouse event handling in
the grab handle view.

Per [Apple's Event Handling
Guide](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/EventOverview/HandlingMouseEvents/HandlingMouseEvents.html):

> Custom NSView objects should not invoke super in their implementations
of NSResponder mouse-event-handling methods such as mouseDown:,
mouseDragged: and mouseUp: unless it is known that the inherited
implementation provides some needed functionality.

This eliminates all state management while solving both the original
issue (#10110) and the new bug.

AI disclosure: claude code found and wrote the fix. I tested it manually
to see that it works. I pressed claude quite hard here to come up with
the best fix, and looked at documentation to understand what the fix was
doing. It seems like this is a better approach overall to preventing the
main window from being dragged when grabbing the Surface Drag handle.

macos/Sources/Ghostty/Surface View/SurfaceDragSource.swift

@@ -105,22 +105,30 @@ extension Ghostty {
         /// Whether the current drag was cancelled by pressing escape.
         private var dragCancelledByEscape: Bool = false
 
-        /// Original value of `window.isMovable` to restore
-        /// when the mouse exits.
-        private var isWindowMovable: Bool?
-
         deinit {
             if let escapeMonitor {
                 NSEvent.removeMonitor(escapeMonitor)
             }
         }
-        
+
+        override func acceptsFirstMouse(for event: NSEvent?) -> Bool {
+            // Ensure this view gets the mouse event before window dragging handlers
+            return true
+        }
+
+        override func mouseDown(with event: NSEvent) {
+            // Consume the mouseDown event to prevent it from propagating to the
+            // window's drag handler. This fixes issue #10110 where grab handles
+            // would drag the window instead of initiating pane drags.
+            // Don't call super - the drag will be initiated in mouseDragged.
+        }
+
         override func updateTrackingAreas() {
             super.updateTrackingAreas()
-            
+
             // To update our tracking area we just recreate it all.
             trackingAreas.forEach { removeTrackingArea($0) }
-            
+
             // Add our tracking area for mouse events
             addTrackingArea(NSTrackingArea(
                 rect: bounds,
@@ -135,18 +143,10 @@ extension Ghostty {
         }
         
         override func mouseEntered(with event: NSEvent) {
-            // Temporarily disable `isMovable` to fix
-            // https://github.com/ghostty-org/ghostty/issues/10110
-            isWindowMovable = window?.isMovable
-            window?.isMovable = false
             onHoverChanged?(true)
         }
         
         override func mouseExited(with event: NSEvent) {
-            if let isWindowMovable {
-                window?.isMovable = isWindowMovable
-                self.isWindowMovable = nil
-            }
             onHoverChanged?(false)
         }
         
@@ -237,7 +237,7 @@ extension Ghostty {
                 NSEvent.removeMonitor(escapeMonitor)
                 self.escapeMonitor = nil
             }
-            
+
             if operation == [] && !dragCancelledByEscape {
                 let endsInWindow = NSApplication.shared.windows.contains { window in
                     window.isVisible && window.frame.contains(screenPoint)
@@ -250,7 +250,7 @@ extension Ghostty {
                     )
                 }
             }
-            
+
             isTracking = false
             onDragStateChanged?(false)
         }