Add split panes feature and fix alias matching

- Split Panes: Cmd+D (vertical) and Cmd+Shift+D (horizontal)
- Close panes with Cmd+W when multiple panes exist
- Fixed alias matching to work with command arguments
- Aliases now match first word only (e.g., 'gs status' works with gs=git)

Split panes allow multiple terminal sessions side-by-side in the same tab

TerminalSession.swift

@@ -402,11 +402,20 @@ class TerminalSession: NSObject, NSTextViewDelegate, NSMenuDelegate {
         
         let trimmed = command.trimmingCharacters(in: .whitespacesAndNewlines)
         
-        // Check for aliases
+        // Check for aliases - match the first word
         var actualCommand = trimmed
-        if let aliasCommand = TerminalSettings.shared.commandAliases[trimmed] {
-            actualCommand = aliasCommand
-            appendText("(alias: \(aliasCommand))\n")
+        let components = trimmed.split(separator: " ", maxSplits: 1)
+        if let firstWord = components.first {
+            let firstWordStr = String(firstWord)
+            if let aliasCommand = TerminalSettings.shared.commandAliases[firstWordStr] {
+                // Replace the alias with the actual command, keeping any arguments
+                if components.count > 1 {
+                    actualCommand = aliasCommand + " " + String(components[1])
+                } else {
+                    actualCommand = aliasCommand
+                }
+                appendText("(alias: \(firstWordStr) → \(aliasCommand))\n")
+            }
         }
         
         // Track directory changes