feat(client): save/load client UI — F5/F6 quicksave/quickload (#554)

Wire SaveGame/LoadGame player actions through the full client stack:
protocol v15 decode, InputMapper F5/F6 bindings, SimBridge wire mapping
with one-shot carry-forward, GameState save_result field, and HUD
notification via monologue display. Quit-to-menu triggers quicksave
before scene change.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/autoloads/session_manager.gd

@@ -84,8 +84,14 @@ func quit_to_menu() -> void:
 
 func _do_quit_to_menu() -> void:
 	_cleanup_quit_dialog()
-	# #554: F5 quicksave will be triggered here before scene change when server
-	# supports SaveCommand. For now: navigate to menu without saving.
+	# #554: Trigger quicksave before navigating to menu.
+	if not GameState.current_game_id.is_empty():
+		var path := "user://saves/" + GameState.current_game_id + "/quicksave.sav"
+		SimBridge.send_input({
+			"action": InputMapper.Action.SAVE_GAME,
+			"timestamp_msec": Time.get_ticks_msec(),
+			"action_data": {"path": path},
+		})
 	GameState.current_game_id = ""
 	get_tree().change_scene_to_file(MENU_SCENE)