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/WorldSyncComponent.gd

@@ -0,0 +1,74 @@
+extends Node
+
+# Attached to ChunkManager node.
+# Server: intercepts block changes and broadcasts to clients.
+# Client: receives and applies block changes locally.
+
+var _chunk_manager: Node = null
+
+
+func _ready() -> void:
+	_chunk_manager = get_parent()
+
+
+# Called by Main.gd instead of world.break_block / world.place_block when in multiplayer.
+func server_break_block(world_pos: Vector3) -> int:
+	if not NetworkManager.is_server():
+		# Client sends request to server
+		request_break.rpc_id(1, world_pos)
+		return 0
+	var broken_id: int = _chunk_manager.break_block(world_pos)
+	if broken_id > 0:
+		broadcast_block_change.rpc(world_pos, 0)
+	return broken_id
+
+
+func server_place_block(world_pos: Vector3, block_id: int) -> bool:
+	if not NetworkManager.is_server():
+		# Client sends request to server
+		request_place.rpc_id(1, world_pos, block_id)
+		return false
+	var placed: bool = _chunk_manager.place_block(world_pos, block_id)
+	if placed:
+		broadcast_block_change.rpc(world_pos, block_id)
+	return placed
+
+
+@rpc("any_peer", "reliable")
+func request_break(pos: Vector3) -> void:
+	if not NetworkManager.is_server():
+		return
+	var sender_id: int = multiplayer.get_remote_sender_id()
+	var broken_id: int = _chunk_manager.break_block(pos)
+	if broken_id > 0:
+		broadcast_block_change.rpc(pos, 0)
+		# Notify requesting peer of item pickup
+		notify_break_result.rpc_id(sender_id, broken_id)
+
+
+@rpc("any_peer", "reliable")
+func request_place(pos: Vector3, block_id: int) -> void:
+	if not NetworkManager.is_server():
+		return
+	var placed: bool = _chunk_manager.place_block(pos, block_id)
+	if placed:
+		broadcast_block_change.rpc(pos, block_id)
+
+
+@rpc("authority", "call_local", "reliable")
+func broadcast_block_change(world_pos: Vector3, new_id: int) -> void:
+	# Applied on all peers including server via call_local
+	if multiplayer.is_server():
+		return  # already applied on server
+	if new_id == 0:
+		_chunk_manager.break_block(world_pos)
+	else:
+		_chunk_manager.place_block(world_pos, new_id)
+
+
+@rpc("authority", "reliable")
+func notify_break_result(broken_id: int) -> void:
+	# Signal to Main.gd to add item to local inventory
+	var main := get_tree().get_first_node_in_group("main")
+	if is_instance_valid(main) and main.has_method("_on_remote_block_broken"):
+		main._on_remote_block_broken(broken_id)