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>

project.godot

@@ -87,10 +87,17 @@ escape={
 ]
 }
 
+open_chat={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":4194332,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+]
+}
+
 [autoload]
 BlockDatabase="*res://scripts/world/BlockDatabase.gd"
 AudioManager="*res://scripts/ambience/AudioManager.gd"
 ItemDatabase="*res://scripts/inventory/ItemDatabase.gd"
+NetworkManager="*res://scripts/net/NetworkManager.gd"
 
 [rendering]
 environment/defaults/default_clear_color=Color(0.05, 0.15, 0.25, 1)