fix: resolve scene dependency errors and missing UID in main.tscn

- WaterBody.tscn, WaterfallBody.tscn: gdshader files were referenced
  with type="Material" instead of type="Shader". Wrapped each
  shader in a ShaderMaterial sub-resource so Godot can load them.
- ChimneySmoke.tscn: reordered sub-resources so StandardMaterial3D
  is defined before QuadMesh that references it.
- WaterfallMist.tscn: fixed node type GpuParticles3D -> GPUParticles3D
  (case-sensitive class name in Godot 4.7).
- jajce_world.gd: added explicit Vector3 type annotation to fix
  'Cannot infer the type of pos variable' parse error.
- main.tscn.uid: created missing UID mapping file. The scene header
  declared uid="uid://rs3hv73svbpa" but no .uid file existed,
  causing Godot to reject the scene as unsupported format.

world/jajce/WaterBody.tscn

@@ -1,9 +1,12 @@
-[gd_scene load_steps=3 format=3]
+[gd_scene load_steps=4 format=3]
 
-[ext_resource type="Material" path="res://world/jajce/materials/water.gdshader" id="1_water_shader"]
+[ext_resource type="Shader" path="res://world/jajce/materials/water.gdshader" id="1_water_shader"]
+
+[sub_resource type="ShaderMaterial" id="ShaderMaterial_water"]
+shader = ExtResource("1_water_shader")
 
 [sub_resource type="BoxMesh" id="Mesh_water_body"]
-material = ExtResource("1_water_shader")
+material = SubResource("ShaderMaterial_water")
 size = Vector3(5, 0.08, 56)
 
 [node name="WaterBody" type="Node3D"]