refactor nodes into standalone file

backend/nodes/image_demo.py

@@ -0,0 +1,37 @@
+from __future__ import annotations
+from backend.node_registry import register_node
+from backend.data_types import COLORMAPS
+from backend.nodes.helpers import DEMO_DIR, _list_demo_files
+
+
+@register_node(display_name="Image (Demo)")
+class ImageDemo:
+    @classmethod
+    def INPUT_TYPES(cls):
+        choices = _list_demo_files() or ["(no demo files found)"]
+        return {
+            "required": {
+                "name": (choices,),
+                "colormap": (list(COLORMAPS), {"hide_when_input_connected": "colormap_map"}),
+            },
+            "optional": {
+                "colormap_map": ("COLORMAP", {"label": "colormap"}),
+            },
+        }
+
+    RETURN_TYPES = ("DATA_FIELD",)
+    RETURN_NAMES = ("field",)
+    FUNCTION = "load"
+
+    DESCRIPTION = "Load a bundled demo file so you can try the app without providing your own data."
+
+    _broadcast_warning_fn = None
+    _current_node_id = None
+
+    def load(self, name: str = "", colormap: str = "viridis", colormap_map=None):
+        from backend.nodes.image import Image
+        loader = Image()
+        demo_path = DEMO_DIR / name
+        if not demo_path.exists():
+            raise FileNotFoundError(f"Demo file not found: {name}")
+        return loader.load(filename=str(demo_path), colormap=colormap, colormap_map=colormap_map)