Fix FOUC and improve Web UI i18n experience

- Fix Flash of Unstyled Content (FOUC) by hiding page until translations load
  - Hide body initially with visibility:hidden
  - Show content after translations are applied to avoid English flash
- Fix authenticated indicator layout with white-space:nowrap
- Add "●" symbol to translation files to prevent disappearance on reload
- Enable network access by binding server to 0.0.0.0 in test config
- Simplify test config API keys to single entry

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

server/src/wled_controller/static/app.js

@@ -132,6 +132,10 @@ function updateAllText() {
 document.addEventListener('DOMContentLoaded', async () => {
     // Initialize locale first
     await initLocale();
+
+    // Show content now that translations are loaded
+    document.body.style.visibility = 'visible';
+
     // Load API key from localStorage
     apiKey = localStorage.getItem('wled_api_key');