Page back/forward: Alt+Left/Right, mouse side buttons, optional on-screen buttons

Alt+Left/Right now go back/forward within the page (the standard browser
keys) instead of switching sites; sites switch with Ctrl+Tab, Ctrl+[ ] and
the standard Ctrl+PageUp/PageDown. New enableHistoryButtons option (off by
default) shows half-transparent back/forward buttons at the top middle.
Settable from the terminal menu and the Web UI.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013wi3ZQveYdjfcrEsyazq32

webui/lib/config.js

@@ -40,6 +40,7 @@ const SCALAR_DEFAULTS = {
     enablePauseButton: true,
     enableKeyboardButton: true,
     enableNavButton: true,
+    enableHistoryButtons: false,
     enablePasswordProtection: false,
     lockoutTimeout: 0,
     lockoutAtTime: '',