fix: rename save-sync.ts → save-sync.svelte.ts for $state compat

Plain .ts files can't read $state proxies from .svelte.ts modules.
The s.subscribe error was caused by SvelteKit treating the raw signal
as a legacy store. Renaming to .svelte.ts fixes the compilation.

Frontend/src/lib/components/GameSync.svelte

@@ -7,7 +7,7 @@
 		startAutoSave,
 		stopAutoSave,
 		setupVisibilitySync,
-	} from '$lib/save-sync';
+	} from '$lib/save-sync.svelte';
 
 	onMount(async () => {
 		// Init auth