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/ChatManager.gd

@@ -0,0 +1,61 @@
+extends Node
+
+# Simple chat: F2 opens input, send to all peers, printed to console.
+
+var _input_open: bool = false
+var _input_line: LineEdit = null
+
+
+func _ready() -> void:
+	# Build minimal LineEdit for chat input
+	_input_line = LineEdit.new()
+	_input_line.placeholder_text = "Message... (Entrée pour envoyer, Echap pour annuler)"
+	_input_line.custom_minimum_size = Vector2(500, 40)
+	_input_line.visible = false
+	_input_line.text_submitted.connect(_on_text_submitted)
+	add_child(_input_line)
+
+	if not InputMap.has_action("open_chat"):
+		InputMap.add_action("open_chat")
+		var ev := InputEventKey.new()
+		ev.physical_keycode = KEY_F2
+		InputMap.action_add_event("open_chat", ev)
+
+
+func _unhandled_input(event: InputEvent) -> void:
+	if event.is_action_pressed("open_chat") and not _input_open:
+		_open_chat()
+		get_viewport().set_input_as_handled()
+	elif _input_open and event.is_action_pressed("escape"):
+		_close_chat()
+		get_viewport().set_input_as_handled()
+
+
+func _open_chat() -> void:
+	_input_open = true
+	_input_line.visible = true
+	_input_line.text = ""
+	_input_line.grab_focus()
+	Input.set_mouse_mode(Input.MOUSE_MODE_VISIBLE)
+
+
+func _close_chat() -> void:
+	_input_open = false
+	_input_line.visible = false
+	Input.set_mouse_mode(Input.MOUSE_MODE_CAPTURED)
+
+
+func _on_text_submitted(text: String) -> void:
+	if text.strip_edges().length() == 0:
+		_close_chat()
+		return
+	send_chat.rpc(text.strip_edges())
+	_close_chat()
+
+
+@rpc("any_peer", "call_local", "reliable")
+func send_chat(msg: String) -> void:
+	var sender_id: int = multiplayer.get_remote_sender_id()
+	if sender_id == 0:
+		sender_id = multiplayer.get_unique_id()
+	print("[CHAT][peer %d] %s" % [sender_id, msg])