feat(multiplayer): ENet networking + player/world sync + lobby menu + chat

Add dedicated server / host / client modes via NetworkManager autoload,
PlayerSyncComponent (20 Hz unreliable RPC), WorldSyncComponent (authoritative
block break/place), ChatManager (F2), LobbyMenu scene, updated MainMenu
with Solo/Heberger/Rejoindre/Quitter buttons. Port changed to 7777
(9999 occupied by sntlkeyssrvr on this machine). Mobs disabled in multi
(spawn solo only). Solo mode untouched.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/net/PlayerSyncComponent.gd

@@ -0,0 +1,63 @@
+extends Node
+
+@export var sync_rate: float = 0.05  # 20 Hz
+
+var _owner_id: int = 1
+var _timer: float = 0.0
+
+# Interpolation buffer for remote dolphins
+var _remote_pos: Vector3 = Vector3.ZERO
+var _remote_yaw: float = 0.0
+var _remote_pitch: float = 0.0
+var _has_remote_data: bool = false
+var _interp_speed: float = 12.0
+
+
+func setup(owner_peer_id: int) -> void:
+	_owner_id = owner_peer_id
+	set_multiplayer_authority(owner_peer_id)
+
+
+func _process(delta: float) -> void:
+	if NetworkManager.is_solo():
+		return
+
+	var parent_node := get_parent()
+	if not is_instance_valid(parent_node):
+		return
+
+	if multiplayer.get_unique_id() == _owner_id:
+		# Send our transform at sync_rate
+		_timer += delta
+		if _timer >= sync_rate:
+			_timer = 0.0
+			var pos: Vector3 = parent_node.global_position
+			var yaw: float = parent_node.rotation.y
+			var pitch: float = 0.0
+			if parent_node.has_node("CameraPivot"):
+				pitch = parent_node.get_node("CameraPivot").rotation.x
+			var boosting: bool = false
+			if "is_boosting" in parent_node:
+				boosting = parent_node.is_boosting
+			receive_transform.rpc(pos, yaw, pitch, boosting)
+	else:
+		# Interpolate toward remote state
+		if _has_remote_data:
+			parent_node.global_position = parent_node.global_position.lerp(
+				_remote_pos, _interp_speed * delta
+			)
+			parent_node.rotation.y = lerp_angle(parent_node.rotation.y, _remote_yaw, _interp_speed * delta)
+			if parent_node.has_node("CameraPivot"):
+				var pivot: Node3D = parent_node.get_node("CameraPivot")
+				pivot.rotation.x = lerp_angle(pivot.rotation.x, _remote_pitch, _interp_speed * delta)
+
+
+@rpc("any_peer", "unreliable")
+func receive_transform(pos: Vector3, rot_yaw: float, rot_pitch: float, boost: bool) -> void:
+	_remote_pos = pos
+	_remote_yaw = rot_yaw
+	_remote_pitch = rot_pitch
+	_has_remote_data = true
+	var parent_node := get_parent()
+	if is_instance_valid(parent_node) and "is_boosting" in parent_node:
+		parent_node.is_boosting = boost