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/api/schemas/system.py

@@ -143,6 +143,20 @@ class MQTTSettingsRequest(BaseModel):
     base_topic: str = Field(default="ledgrab", description="Base topic prefix")
 
 
+# ─── External URL schema ───────────────────────────────────────
+
+class ExternalUrlResponse(BaseModel):
+    """External URL setting response."""
+
+    external_url: str = Field(description="External base URL (e.g. https://myserver.example.com:8080). Empty = use auto-detected URL.")
+
+
+class ExternalUrlRequest(BaseModel):
+    """External URL setting update request."""
+
+    external_url: str = Field(default="", description="External base URL. Empty string to clear.")
+
+
 # ─── Log level schemas ─────────────────────────────────────────
 
 class LogLevelResponse(BaseModel):