PC3 Phase 1.4-1.6: avatars wired into the game end to end

- api_client.gd: get_avatar_loadout() / save_avatar_loadout()
- main_menu.gd: on successful auth (fresh or session-restore), load the
  catalog + fetch the saved loadout into AvatarSystem so the in-world
  player renders as the player's avatar. Best-effort: a slow/failed prime
  never gates getting in (UI goes authenticated first, prime runs after).
  New "Customize Avatar" button (enabled only when signed in).
- avatar_system.gd: local-player convenience API -- ensure_catalog(),
  set_loadout_from_dict() (from the backend's GET shape), has_loadout(),
  build_current_avatar().
- player.gd: swaps the placeholder capsule for the assembled avatar
  (feet-aligned to the capsule bottom) when a loadout is primed; silent
  fallback to the capsule otherwise. Camera stays first-person for now
  (third-person is a noted follow-up).
- avatar_visualizer.gd: rewrote it -- the old version referenced
  AvatarLoader.SlotResult / load_slot() which never existed, so it didn't
  compile and took avatar_customization_ui.gd down with it. Now a thin
  wrapper over AvatarLoader for the preview.
- avatar_customization_ui.gd: seeds from the live loadout, saves through to
  AvatarSystem + PUT /api/avatar/loadout (local file kept as fallback),
  fixed a Camera3D.look_at() before-in-tree crash.
- scenes/avatar_customization.tscn rebuilt (old one used the Godot-3
  `ViewportContainer` node type) + avatar_customization_screen.gd wrapper
  that returns to the menu on close.
- smoke_test_avatar_in_world.gd: primed loadout -> game.tscn -> player
  shows the assembled avatar, capsule hidden, rig + bone mounts present.

Verified: cold-cache smokes green (player 15/15, avatar_system 14/14,
avatar_in_world 10/10, main_menu_ui 13/13); against a live backend
main_menu_full_flow 12/12, main_menu_back 20/20, world_list 4/4,
enter_world 9/9, e2e 4/4; plus a manual register->default-loadout->
customize->save->re-fetch-persisted round trip.

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

scripts/player.gd

@@ -32,7 +32,14 @@ const PITCH_LIMIT: float = deg_to_rad(89.0)
 var _input_direction := Vector2.ZERO
 var _camera_pitch: float = 0.0
 
+## Vertical offset applied to the assembled avatar so its feet (origin y=0)
+## line up with the bottom of the player's collision capsule.
+const AVATAR_FEET_OFFSET: float = -0.8
+
 @onready var _camera: Camera3D = $Camera3D
+@onready var _capsule_mesh: MeshInstance3D = $CapsuleMesh
+
+var _avatar_view: Node3D = null
 
 
 # ---------------------------------------------------------------------------
@@ -67,6 +74,28 @@ func _ready() -> void:
 	if DisplayServer.get_name() != "headless":
 		Input.mouse_mode = Input.MOUSE_MODE_CAPTURED
 
+	_apply_avatar()
+
+
+## Replace the placeholder capsule with the player's assembled avatar, if the
+## AvatarSystem has a loadout primed (set at login by main_menu.gd). Falls back
+## to the capsule silently when there's no avatar to show -- e.g. running this
+## scene directly in a test, or an offline/failed catalog load.
+## TODO(PC3): a third-person camera so the local player actually sees this;
+## for now it's mainly for other players (Phase 2) and looking down.
+func _apply_avatar() -> void:
+	var avatars := get_node_or_null("/root/AvatarSystem")
+	if avatars == null or not avatars.has_method("build_current_avatar"):
+		return
+	var avatar: Node3D = avatars.build_current_avatar()
+	if avatar == null:
+		return
+	avatar.position = Vector3(0.0, AVATAR_FEET_OFFSET, 0.0)
+	add_child(avatar)
+	_avatar_view = avatar
+	if _capsule_mesh:
+		_capsule_mesh.visible = false
+
 
 # ---------------------------------------------------------------------------
 # _unhandled_input — capture keyboard / mouse for movement