Add keyboard shortcut to close split panes

- Cmd+Shift+W closes the active pane
- Automatically removes split view when only one pane remains
- Falls back to closing tab if no splits exist
- Updated keyboard shortcuts reference

MainWindowController.swift

@@ -158,7 +158,11 @@ class MainWindowController: NSWindowController {
         fileMenu.addItem(NSMenuItem.separator())
         fileMenu.addItem(NSMenuItem(title: "Split Pane Vertically", action: #selector(splitPaneVertically), keyEquivalent: "d"))
         fileMenu.addItem(NSMenuItem(title: "Split Pane Horizontally", action: #selector(splitPaneHorizontally), keyEquivalent: "D"))
-        fileMenu.addItem(NSMenuItem(title: "Close Pane", action: #selector(closePane), keyEquivalent: "w"))
+        
+        let closePaneItem = NSMenuItem(title: "Close Pane", action: #selector(closePane), keyEquivalent: "w")
+        closePaneItem.keyEquivalentModifierMask = [.command, .shift]
+        fileMenu.addItem(closePaneItem)
+        
         fileMenu.addItem(NSMenuItem.separator())
         fileMenu.addItem(NSMenuItem(title: "Clear Screen", action: #selector(clearScreen), keyEquivalent: "k"))
         fileMenu.addItem(NSMenuItem(title: "Clear Scrollback", action: #selector(clearScrollback), keyEquivalent: "K"))
@@ -509,6 +513,7 @@ class MainWindowController: NSWindowController {
           Cmd+W - Close Tab
           Cmd+D - Split Pane Vertically
           Cmd+Shift+D - Split Pane Horizontally
+          Cmd+Shift+W - Close Pane
           Cmd+K - Clear Screen
           Cmd+Shift+K - Clear Scrollback
         
@@ -625,12 +630,25 @@ class MainWindowController: NSWindowController {
     }
     
     @objc func closePane() {
-        guard panes.count > 1, activePaneIndex < panes.count else { return }
+        // Only close panes if we have splits
+        guard panes.count > 1 else {
+            // No splits, just close the tab normally
+            closeCurrentTab()
+            return
+        }
         
-        let pane = panes[activePaneIndex]
+        // Find which pane has focus
+        var paneToClose = activePaneIndex
+        
+        // If we can't determine active pane, close the last one
+        if paneToClose >= panes.count {
+            paneToClose = panes.count - 1
+        }
+        
+        let pane = panes[paneToClose]
         pane.cleanup()
         pane.scrollView.removeFromSuperview()
-        panes.remove(at: activePaneIndex)
+        panes.remove(at: paneToClose)
         
         if panes.count == 1 {
             // Only one pane left, remove split view
@@ -641,10 +659,15 @@ class MainWindowController: NSWindowController {
             remainingSession.scrollView.removeFromSuperview()
             remainingSession.scrollView.frame = contentView.bounds
             contentView.addSubview(remainingSession.scrollView)
+            remainingSession.textView.window?.makeFirstResponder(remainingSession.textView)
             panes = []
+        } else {
+            // Still have multiple panes
+            activePaneIndex = min(paneToClose, panes.count - 1)
+            if activePaneIndex < panes.count {
+                panes[activePaneIndex].textView.window?.makeFirstResponder(panes[activePaneIndex].textView)
+            }
         }
-        
-        activePaneIndex = min(activePaneIndex, panes.count - 1)
     }
     
     @objc func clearHistory() {