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/test/config.test.js

@@ -81,6 +81,9 @@ check('enableHistoryButtons defaults off and round-trips', () => {
     saveConfig({ enableHistoryButtons: true });
     assert.strictEqual(JSON.parse(fs.readFileSync(process.env.CONFIG_PATH, 'utf8')).enableHistoryButtons, true);
     assert.strictEqual(loadConfig().enableHistoryButtons, true);
+    assert.strictEqual(loadConfig().historyButtonsOpacity, 50);
+    saveConfig({ historyButtonsOpacity: 80 });
+    assert.strictEqual(loadConfig().historyButtonsOpacity, 80);
 });
 
 check('saveConfig tabs: new password gets hashed, never stored/returned as plaintext', () => {