fix(client): rename stores/game.ts → game.svelte.ts so Svelte 5 runes compile

Svelte 5 runes ($state, $derived, $effect) only run through the
compiler in .svelte and .svelte.ts/js files. A plain .ts file leaves
$state(...) as a literal call at runtime, causing
"ReferenceError: $state is not defined" and a blank page.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/client/src/lib/Game.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { game } from './stores/game.js';
+  import { game } from './stores/game.svelte.js';
   import Board from './Board.svelte';
   import ModeratorPanel from './ModeratorPanel.svelte';
   import PromotionDialog from './PromotionDialog.svelte';