fix: resolve Svelte 5 effect_update_depth_exceeded infinite loop

- ScoreBoard: wrap floats mutation in untrack() to prevent $effect
  from re-triggering when it reads+writes floats in same cycle
- App: split bestScore effect and use untrack() to prevent read/write
  loop on bestScore within the same $effect

src/App.svelte

@@ -2,6 +2,7 @@
   import Grid from './components/Grid.svelte';
   import ScoreBoard from './components/ScoreBoard.svelte';
   import GameMessage from './components/GameMessage.svelte';
+  import { untrack } from 'svelte';
   import { initGame, move, isGameOver } from './lib/game-logic.js';
   import { GRID_SIZE } from './lib/constants.js';
   import { getDirectionFromKey, getDirectionFromSwipe } from './lib/input-handler.js';
@@ -27,9 +28,14 @@
   });
 
   $effect(() => {
-    if (gameState.score > bestScore) {
-      bestScore = gameState.score;
+    const score = gameState.score;
+    const best = untrack(() => bestScore);
+    if (score > best) {
+      bestScore = score;
     }
+  });
+
+  $effect(() => {
     saveBestScore(bestScore);
   });
 

src/components/ScoreBoard.svelte

@@ -1,4 +1,6 @@
 <script>
+  import { untrack } from 'svelte';
+
   let { score = 0, bestScore = 0, scoreDelta = 0 } = $props();
 
   let floats = $state([]);
@@ -7,7 +9,10 @@
   $effect(() => {
     if (scoreDelta > 0) {
       const id = ++floatId;
-      floats = [...floats, { id, value: scoreDelta }];
+      const delta = scoreDelta;
+      untrack(() => {
+        floats = [...floats, { id, value: delta }];
+      });
       setTimeout(() => {
         floats = floats.filter(f => f.id !== id);
       }, 600);