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/css/tree-nav.css

@@ -99,6 +99,17 @@
     text-align: center;
 }
 
+/* ── Nested sub-group (group inside a group) ── */
+
+.tree-group-nested > .tree-group-header {
+    font-size: 0.75rem;
+    text-transform: none;
+    letter-spacing: normal;
+    font-weight: 600;
+    margin-top: 2px;
+    padding: 4px 10px 4px 12px;
+}
+
 /* ── Children (leaves) ── */
 
 .tree-children {