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>

scenes/LobbyMenu.tscn

@@ -0,0 +1,107 @@
+[gd_scene load_steps=3 format=3 uid="uid://dauphincraft_lobby"]
+
+[ext_resource type="Script" path="res://scripts/net/LobbyMenu.gd" id="1_lobbyscript"]
+
+[sub_resource type="StyleBoxFlat" id="StyleBoxFlat_btn"]
+bg_color = Color(0.05, 0.15, 0.3, 0.75)
+border_width_left = 2
+border_width_top = 2
+border_width_right = 2
+border_width_bottom = 2
+border_color = Color(0.3, 0.8, 1.0, 1.0)
+corner_radius_top_left = 6
+corner_radius_top_right = 6
+corner_radius_bottom_right = 6
+corner_radius_bottom_left = 6
+
+[node name="LobbyMenu" type="Control"]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+script = ExtResource("1_lobbyscript")
+
+[node name="Background" type="ColorRect" parent="."]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+color = Color(0.02, 0.08, 0.18, 1.0)
+
+[node name="Panel" type="VBoxContainer" parent="."]
+layout_mode = 1
+anchors_preset = 8
+anchor_left = 0.5
+anchor_top = 0.5
+anchor_right = 0.5
+anchor_bottom = 0.5
+offset_left = -200.0
+offset_top = -200.0
+offset_right = 200.0
+offset_bottom = 200.0
+alignment = 1
+
+[node name="Title" type="Label" parent="Panel"]
+layout_mode = 2
+text = "REJOINDRE"
+horizontal_alignment = 1
+theme_override_font_sizes/font_size = 48
+theme_override_colors/font_color = Color(0.7, 0.95, 1.0, 1.0)
+
+[node name="Spacer1" type="Control" parent="Panel"]
+layout_mode = 2
+custom_minimum_size = Vector2(0, 20)
+
+[node name="VBox" type="VBoxContainer" parent="Panel"]
+layout_mode = 2
+
+[node name="IPLabel" type="Label" parent="Panel/VBox"]
+text = "Adresse IP du serveur"
+theme_override_font_sizes/font_size = 18
+theme_override_colors/font_color = Color(0.7, 0.9, 1.0, 1.0)
+
+[node name="IPInput" type="LineEdit" parent="Panel/VBox"]
+layout_mode = 2
+custom_minimum_size = Vector2(400, 48)
+placeholder_text = "127.0.0.1"
+theme_override_font_sizes/font_size = 20
+
+[node name="Spacer2" type="Control" parent="Panel/VBox"]
+layout_mode = 2
+custom_minimum_size = Vector2(0, 10)
+
+[node name="PortLabel" type="Label" parent="Panel/VBox"]
+text = "Port"
+theme_override_font_sizes/font_size = 18
+theme_override_colors/font_color = Color(0.7, 0.9, 1.0, 1.0)
+
+[node name="PortInput" type="LineEdit" parent="Panel/VBox"]
+layout_mode = 2
+custom_minimum_size = Vector2(400, 48)
+placeholder_text = "9999"
+theme_override_font_sizes/font_size = 20
+
+[node name="Spacer3" type="Control" parent="Panel/VBox"]
+layout_mode = 2
+custom_minimum_size = Vector2(0, 10)
+
+[node name="StatusLabel" type="Label" parent="Panel/VBox"]
+layout_mode = 2
+theme_override_font_sizes/font_size = 16
+theme_override_colors/font_color = Color(1.0, 0.8, 0.3, 1.0)
+
+[node name="ConnectBtn" type="Button" parent="Panel/VBox"]
+layout_mode = 2
+custom_minimum_size = Vector2(400, 56)
+text = "Se connecter"
+theme_override_font_sizes/font_size = 24
+theme_override_colors/font_color = Color(0.8, 0.97, 1.0, 1.0)
+theme_override_styles/normal = SubResource("StyleBoxFlat_btn")
+
+[node name="BackBtn" type="Button" parent="Panel/VBox"]
+layout_mode = 2
+custom_minimum_size = Vector2(400, 56)
+text = "Retour"
+theme_override_font_sizes/font_size = 24
+theme_override_colors/font_color = Color(0.8, 0.97, 1.0, 1.0)
+theme_override_styles/normal = SubResource("StyleBoxFlat_btn")