experimental: remove Tekton Doors entirely

- Delete portal_mode_manager.gd, portal_door.gd, portal_door.tscn
- Strip all Tekton Doors logic from main.gd, player.gd, lobby.gd,
  lobby_room.gd, lobby_manager.gd, camera_context_manager.gd,
  music_manager.gd, tekton.gd, enhanced_gridmap.gd,
  playerboard_manager.gd, special_tiles_manager.gd
- Remove TK enum (TEKTON_DOORS=2), mode_config schema, arena area
- Update tests: 3 modes instead of 4
- Strip HowToPlay tab from main.tscn

scripts/tekton.gd

@@ -514,9 +514,7 @@ func spawn_tiles_around(count: int = 4):
 			if LobbyManager:
 				mode = LobbyManager.get_game_mode()
 			
-			if LobbyManager and LobbyManager.get_game_mode() == GameMode.Mode.TEKTON_DOORS:
-				item_id = [11, 14].pick_random()
-			elif mode == GameMode.Mode.FREEMODE:
+			if mode == GameMode.Mode.FREEMODE:
 				item_id = rng.randi_range(7, 10) # No powerups in freemode either, just normal tiles
 			else:
 				item_id = rng.randi_range(11, 14)