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/LobbyMenu.gd

@@ -0,0 +1,44 @@
+extends Control
+
+@onready var _ip_input: LineEdit = $Panel/VBox/IPInput
+@onready var _port_input: LineEdit = $Panel/VBox/PortInput
+@onready var _status_label: Label = $Panel/VBox/StatusLabel
+@onready var _connect_btn: Button = $Panel/VBox/ConnectBtn
+@onready var _back_btn: Button = $Panel/VBox/BackBtn
+
+
+func _ready() -> void:
+	_ip_input.text = "127.0.0.1"
+	_port_input.text = str(NetworkManager.DEFAULT_PORT)
+	_status_label.text = ""
+
+	NetworkManager.connection_succeeded.connect(_on_connected)
+	NetworkManager.connection_failed.connect(_on_failed)
+
+	_connect_btn.pressed.connect(_on_connect_pressed)
+	_back_btn.pressed.connect(_on_back_pressed)
+
+
+func _on_connect_pressed() -> void:
+	var address: String = _ip_input.text.strip_edges()
+	var port: int = int(_port_input.text.strip_edges())
+	if address.length() == 0:
+		_status_label.text = "Adresse invalide"
+		return
+	_status_label.text = "Connexion en cours..."
+	_connect_btn.disabled = true
+	NetworkManager.join_server(address, port)
+
+
+func _on_connected() -> void:
+	_status_label.text = "Connecte!"
+	get_tree().change_scene_to_file("res://scenes/Main.tscn")
+
+
+func _on_failed() -> void:
+	_status_label.text = "Connexion echouee"
+	_connect_btn.disabled = false
+
+
+func _on_back_pressed() -> void:
+	get_tree().change_scene_to_file("res://scenes/MainMenu.tscn")