fix(client): address PR #73 review — race conditions and defensive guards (#257)

- Defer LOAD_GAME dispatch until SimBridge reaches CONNECTED (critical)
- Guard _build_saves_list() against queue_free() race on rapid reopen
- Disable save entries with empty newest_save, guard in _on_save_selected
- Send before show_loading on F6 quickload, skip overlay on send failure
- Clear pending_load_path in _on_new_game()/_on_continue() (stale path)
- Add hide_loading(success: bool) API for future failure-state UI
- Add test_save_load_flow_sprint21.gd covering LoadingScreen + GameState

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

client/ui/loading_screen.gd

@@ -39,5 +39,6 @@ func show_loading() -> void:
 	visible = true
 
 
-func hide_loading() -> void:
+## Hide the loading overlay. success=false is reserved for future failure-state UI.
+func hide_loading(success: bool = true) -> void:
 	visible = false