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/templates/modals/css-editor.html

@@ -567,7 +567,7 @@
                 <div id="css-editor-processed-section" style="display:none">
                     <div class="form-group">
                         <div class="label-row">
-                            <label for="css-editor-processed-input" data-i18n="color_strip.processed.input">Input Source:</label>
+                            <label for="css-editor-processed-input" data-i18n="color_strip.processed.input">Source:</label>
                             <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
                         </div>
                         <small class="input-hint" style="display:none" data-i18n="color_strip.processed.input.hint">The color strip source whose output will be processed</small>