Third-person camera + fix the avatar customize preview

You couldn't see your own character because the camera was first-person
(inside the avatar's head), and the customize screen's 3D preview rendered
nothing.

- player.tscn: camera now hangs off a SpringArm3D on a CameraPivot, 4 m
  behind + above the player, pulled in on collision. player.gd: yaw still
  turns the body, pitch orbits the arm (starts tilted down to frame the
  avatar).
- avatar_customization_ui.gd: the preview SubViewport had own_world_3d=false,
  so it shared the Control-only main world -- no scenario, nothing rendered.
  Set own_world_3d=true, add a WorldEnvironment with ambient fill + a
  key/fill light pair, make the camera current, tweak framing.
- smoke_test_player: assert the third-person SpringArm rig (18/18).
- smoke_test_enter_world: wait longer for the scene change -- a real
  POST /join spawns a dedicated server, which is slow (9/9).

Client-side change -- rebuild the exe to get it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

scenes/player.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=3 format=3 uid="uid://player_scene_001"]
+[gd_scene load_steps=4 format=3 uid="uid://player_scene_001"]
 
 [ext_resource type="Script" path="res://scripts/player.gd" id="1_player"]
 
@@ -19,6 +19,13 @@ shape = SubResource("CapsuleShape1")
 [node name="CapsuleMesh" type="MeshInstance3D" parent="."]
 mesh = SubResource("CapsuleMesh1")
 
-[node name="Camera3D" type="Camera3D" parent="."]
-transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0.6, 0)
+[node name="CameraPivot" type="Node3D" parent="."]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0.7, 0)
+
+[node name="SpringArm3D" type="SpringArm3D" parent="CameraPivot"]
+transform = Transform3D(-1, 0, -8.7422777e-08, 0, 1, 0, 8.7422777e-08, 0, -1, 0, 0, 0)
+spring_length = 4.0
+margin = 0.2
+
+[node name="Camera3D" type="Camera3D" parent="CameraPivot/SpringArm3D"]
 current = true