fix: change split-down shortcut to Alt+S to avoid system conflict

- Change split-down from Alt+Shift+D to Alt+S (Alt+Shift is Windows
  keyboard layout switch shortcut)
- Use event.code for keyboard-layout-independent key detection
- Add red theme to dark theme scrollbar selectors
- Add red-themed scrollbar styling with dark red colors
- Tone down white/bright colors in red terminal theme

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/app/src/store/app-store.ts

@@ -199,7 +199,7 @@ export const DEFAULT_KEYBOARD_SHORTCUTS: KeyboardShortcuts = {
   // Terminal shortcuts (only active in terminal view)
   // Using Alt modifier to avoid conflicts with both terminal signals AND browser shortcuts
   splitTerminalRight: "Alt+D",
-  splitTerminalDown: "Alt+Shift+D",
+  splitTerminalDown: "Alt+S",
   closeTerminal: "Alt+W",
 };