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/test-css-source.html

@@ -46,7 +46,7 @@
 
             <!-- CSPT test: input source selector (hidden by default) -->
             <div id="css-test-cspt-input-group" style="display:none" class="css-test-led-control">
-                <label for="css-test-cspt-input-select" data-i18n="color_strip.processed.input">Input Source:</label>
+                <label for="css-test-cspt-input-select" data-i18n="color_strip.processed.input">Source:</label>
                 <select id="css-test-cspt-input-select" class="css-test-cspt-select" onchange="applyCssTestSettings()"></select>
             </div>