Settings tabs, log overlay, external URL, Sources tree restructure, audio fixes

- Settings modal split into 3 tabs: General, Backup, MQTT
- Log viewer moved to full-screen overlay with compact toolbar
- External URL setting: API endpoints + UI for configuring server domain
  used in webhook/WS URLs instead of auto-detected local IP
- Sources tab tree restructured: Picture Source (Screen Capture/Static/
  Processed sub-groups), Color Strip, Audio, Utility
- TreeNav extended to support nested groups (3-level tree)
- Audio tab split into Sources and Templates sub-tabs
- Fix audio template test: device picker now filters by engine type
  (was showing WASAPI indices for sounddevice templates)
- Audio template test device picker disabled during active test
- Rename "Input Source" to "Source" in CSS test preview (en/ru/zh)
- Fix i18n: log filter/level items deferred to avoid stale t() calls

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/static/js/app.js

@@ -191,6 +191,7 @@ import {
     connectLogViewer, disconnectLogViewer, clearLogViewer, applyLogFilter,
     openLogOverlay, closeLogOverlay,
     loadLogLevel, setLogLevel,
+    saveExternalUrl, getBaseOrigin, loadExternalUrl,
 } from './features/settings.js';
 
 // ─── Register all HTML onclick / onchange / onfocus globals ───
@@ -547,6 +548,8 @@ Object.assign(window, {
     closeLogOverlay,
     loadLogLevel,
     setLogLevel,
+    saveExternalUrl,
+    getBaseOrigin,
 });
 
 // ─── Global keyboard shortcuts ───
@@ -613,6 +616,9 @@ document.addEventListener('DOMContentLoaded', async () => {
     // Initialize locale (dispatches languageChanged which may trigger API calls)
     await initLocale();
 
+    // Load external URL setting early so getBaseOrigin() is available for card rendering
+    loadExternalUrl();
+
     // Restore active tab before showing content to avoid visible jump
     initTabs();