Back/forward buttons: adjustable opacity; Upgrade refreshes the Web UI copy

New historyButtonsOpacity setting (10-100 percent, default 50) in the
terminal menu (Display & Interaction) and the Web UI. Upgrade now also
copies the Web UI app to /opt/kiosk-webui and restarts it, so new Web UI
settings appear after an upgrade (it previously kept its old copy).

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

webui/lib/config.js

@@ -41,6 +41,7 @@ const SCALAR_DEFAULTS = {
     enableKeyboardButton: true,
     enableNavButton: true,
     enableHistoryButtons: false,
+    historyButtonsOpacity: 50,
     enablePasswordProtection: false,
     lockoutTimeout: 0,
     lockoutAtTime: '',