feat: add chase and gradient flash notification effects with priority queue

New notification effects:
- Chase: light bounces across strip with Gaussian glow tail
- Gradient flash: bright center fades to edges with exponential decay

Queue priority: notifications with color_override get high priority and
interrupt the current effect.

Also fixes transient preview for notification sources — adds WebSocket
"fire" command so inline preview works without a saved source, plus
auto-fires on preview open so the effect is visible immediately.

server/src/wled_controller/api/routes/color_strip_sources.py

@@ -507,7 +507,7 @@ async def os_notification_history(_auth: AuthRequired):
 
 # ── Transient Preview WebSocket ────────────────────────────────────────
 
-_PREVIEW_ALLOWED_TYPES = {"static", "gradient", "color_cycle", "effect", "daylight", "candlelight"}
+_PREVIEW_ALLOWED_TYPES = {"static", "gradient", "color_cycle", "effect", "daylight", "candlelight", "notification"}
 
 
 @router.websocket("/api/v1/color-strip-sources/preview/ws")
@@ -648,6 +648,17 @@ async def preview_color_strip_ws(
             if msg is not None:
                 try:
                     new_config = _json.loads(msg)
+
+                    # Handle "fire" command for notification streams
+                    if new_config.get("action") == "fire":
+                        from wled_controller.core.processing.notification_stream import NotificationColorStripStream
+                        if isinstance(stream, NotificationColorStripStream):
+                            stream.fire(
+                                app_name=new_config.get("app", ""),
+                                color_override=new_config.get("color"),
+                            )
+                        continue
+
                     new_type = new_config.get("source_type")
                     if new_type not in _PREVIEW_ALLOWED_TYPES:
                         await websocket.send_text(_json.dumps({"type": "error", "detail": f"source_type must be one of {sorted(_PREVIEW_ALLOWED_TYPES)}"}))