Fix split pane orientations

- Cmd+D now correctly creates vertical split (side-by-side)
- Cmd+Shift+D creates horizontal split (top-bottom)
- Recreates split view if changing orientation
- Clear distinction between the two split types

MainWindowController.swift

@@ -543,10 +543,18 @@ class MainWindowController: NSWindowController {
     @objc func splitPaneVertically() {
         guard let currentSession = tabManager.getCurrentSession() else { return }
         
+        // If split view exists with wrong orientation, recreate it
+        if let existing = splitView, !existing.isVertical {
+            // Remove and recreate with correct orientation
+            existing.removeFromSuperview()
+            splitView = nil
+            panes = []
+        }
+        
         // Create split view if it doesn't exist
         if splitView == nil {
             splitView = NSSplitView(frame: contentView.bounds)
-            splitView?.isVertical = true
+            splitView?.isVertical = true  // Side-by-side (vertical divider)
             splitView?.dividerStyle = .thin
             splitView?.autoresizingMask = [.width, .height]
             
@@ -577,10 +585,18 @@ class MainWindowController: NSWindowController {
     @objc func splitPaneHorizontally() {
         guard let currentSession = tabManager.getCurrentSession() else { return }
         
+        // If split view exists with wrong orientation, recreate it
+        if let existing = splitView, existing.isVertical {
+            // Remove and recreate with correct orientation
+            existing.removeFromSuperview()
+            splitView = nil
+            panes = []
+        }
+        
         // Create split view if it doesn't exist
         if splitView == nil {
             splitView = NSSplitView(frame: contentView.bounds)
-            splitView?.isVertical = false
+            splitView?.isVertical = false  // Top-bottom (horizontal divider)
             splitView?.dividerStyle = .thin
             splitView?.autoresizingMask = [.width, .height]