Add floating close button to split panes with subtle design

- Added HoverCloseButton class with hover effects
- Close button appears in top-right corner of each split pane
- Subtle dark semi-transparent design that brightens on hover
- Container view approach for proper layout and positioning
- Updated help and keyboard shortcuts documentation
- Removed non-working Cmd+Shift+W from help displays

TerminalSession.swift

@@ -568,6 +568,8 @@ class TerminalSession: NSObject, NSTextViewDelegate, NSMenuDelegate {
         Keyboard Shortcuts:
           • Cmd+T          - New tab
           • Cmd+W          - Close tab
+          • Cmd+D          - Split pane vertically (side-by-side)
+          • Cmd+Shift+D    - Split pane horizontally (top-bottom)
           • Cmd+R          - Search command history (fuzzy search)
           • Cmd+K          - AI command generator
           • Cmd+P          - Command snippets