Add quick-win features: copy-on-select, middle-click paste, visual bell, zoom pane, clickable URLs, draggable dividers

- Copy on select: Auto-copy selected text to clipboard
- Middle-click paste: Quick paste with middle mouse button
- Visual bell: Subtle flash when commands complete
- Zoom pane: Cmd+Shift+Enter to maximize/restore split pane
- Clickable URLs: Cmd+click to open URLs in browser
- Draggable dividers: Resize split panes by dragging divider
- Updated help and keyboard shortcuts documentation

Settings.swift

@@ -17,6 +17,8 @@ class TerminalSettings {
     var maxHistorySize: Int = 1000
     var startupCommands: [String] = []
     var commandAliases: [String: String] = [:]  // alias -> command
+    var copyOnSelect: Bool = true
+    var visualBell: Bool = true
     
     enum AIProvider: String, CaseIterable {
         case none = "None"