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/settings.html

@@ -26,6 +26,19 @@
                     <div id="settings-api-keys-list" style="font-size:0.85rem;"></div>
                 </div>
 
+                <!-- External URL -->
+                <div class="form-group">
+                    <div class="label-row">
+                        <label data-i18n="settings.external_url.label">External URL</label>
+                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
+                    </div>
+                    <small class="input-hint" style="display:none" data-i18n="settings.external_url.hint">If set, this base URL is used in webhook URLs and other user-visible links instead of the auto-detected local IP. Example: https://myserver.example.com:8080</small>
+                    <div style="display:flex;gap:0.5rem;">
+                        <input type="text" id="settings-external-url" placeholder="https://myserver.example.com:8080" style="flex:1" data-i18n-placeholder="settings.external_url.placeholder">
+                        <button class="btn btn-primary" onclick="saveExternalUrl()" data-i18n="settings.external_url.save">Save</button>
+                    </div>
+                </div>
+
                 <!-- Log Level section -->
                 <div class="form-group">
                     <div class="label-row">