Add floating close button to split panes

- Each pane now has a floating × button in top-right corner
- Semi-transparent black background with white text
- Tooltip shows keyboard shortcut
- Auto-positions when resizing
- Makes closing panes more intuitive

MainWindowController.swift

@@ -565,6 +565,7 @@ class MainWindowController: NSWindowController {
             
             // Move current session to split view
             currentSession.scrollView.removeFromSuperview()
+            addCloseButtonToPane(currentSession.scrollView, index: 0)
             splitView?.addArrangedSubview(currentSession.scrollView)
             panes = [currentSession]
         }
@@ -574,6 +575,7 @@ class MainWindowController: NSWindowController {
         newSession.setupTextView(frame: CGRect(x: 0, y: 0, width: contentView.bounds.width / 2, height: contentView.bounds.height))
         newSession.startShell()
         
+        addCloseButtonToPane(newSession.scrollView, index: panes.count)
         splitView?.addArrangedSubview(newSession.scrollView)
         panes.append(newSession)
         activePaneIndex = panes.count - 1
@@ -607,6 +609,7 @@ class MainWindowController: NSWindowController {
             
             // Move current session to split view
             currentSession.scrollView.removeFromSuperview()
+            addCloseButtonToPane(currentSession.scrollView, index: 0)
             splitView?.addArrangedSubview(currentSession.scrollView)
             panes = [currentSession]
         }
@@ -616,6 +619,7 @@ class MainWindowController: NSWindowController {
         newSession.setupTextView(frame: CGRect(x: 0, y: 0, width: contentView.bounds.width, height: contentView.bounds.height / 2))
         newSession.startShell()
         
+        addCloseButtonToPane(newSession.scrollView, index: panes.count)
         splitView?.addArrangedSubview(newSession.scrollView)
         panes.append(newSession)
         activePaneIndex = panes.count - 1
@@ -629,6 +633,41 @@ class MainWindowController: NSWindowController {
         newSession.textView.window?.makeFirstResponder(newSession.textView)
     }
     
+    private func addCloseButtonToPane(_ scrollView: NSScrollView, index: Int) {
+        // Remove any existing close button
+        scrollView.subviews.filter { $0.tag == 9999 }.forEach { $0.removeFromSuperview() }
+        
+        // Create close button
+        let closeButton = NSButton(frame: NSRect(x: scrollView.bounds.width - 30, y: scrollView.bounds.height - 30, width: 24, height: 24))
+        closeButton.title = "×"
+        closeButton.bezelStyle = .circular
+        closeButton.font = NSFont.systemFont(ofSize: 16, weight: .bold)
+        closeButton.tag = 9999  // Special tag to identify close buttons
+        closeButton.target = self
+        closeButton.action = #selector(closePaneFromButton(_:))
+        closeButton.toolTip = "Close pane (Cmd+Shift+W)"
+        closeButton.autoresizingMask = [.minXMargin, .minYMargin]
+        closeButton.wantsLayer = true
+        closeButton.layer?.backgroundColor = NSColor.black.withAlphaComponent(0.5).cgColor
+        closeButton.layer?.cornerRadius = 12
+        closeButton.contentTintColor = .white
+        
+        // Store the pane index
+        objc_setAssociatedObject(closeButton, "paneIndex", index, .OBJC_ASSOCIATION_RETAIN)
+        
+        scrollView.addSubview(closeButton)
+    }
+    
+    @objc func closePaneFromButton(_ sender: NSButton) {
+        guard let index = objc_getAssociatedObject(sender, "paneIndex") as? Int else { return }
+        
+        // Find the actual current index (it might have changed)
+        guard index < panes.count else { return }
+        
+        activePaneIndex = index
+        closePane()
+    }
+    
     @objc func closePane() {
         // Only close panes if we have splits
         guard panes.count > 1 else {