feat: ディザリングのデフォルトをFloyd-Steinbergに変更

- 最も汎用性が高く自然な結果が得られるため
- ほとんどの画像タイプに対応
- リセット時もFloyd-Steinbergに戻るように設定

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: ayutaz

index.html

@@ -111,7 +111,7 @@ <h2 class="section-title">変換設定</h2>
                         <div class="setting-control">
                             <select id="dithering" class="select">
                                 <option value="none">なし</option>
-                                <option value="floyd-steinberg">Floyd-Steinberg</option>
+                                <option value="floyd-steinberg" selected>Floyd-Steinberg</option>
                                 <option value="ordered">Ordered (Bayer)</option>
                                 <option value="atkinson">Atkinson</option>
                             </select>

src/js/app-v3.js

@@ -1344,7 +1344,7 @@
             document.getElementById('pixelSize').value = 8;
             document.getElementById('pixelSizeValue').textContent = '8';
             document.getElementById('outputWidth').value = 512;
-            document.getElementById('dithering').value = 'none';
+            document.getElementById('dithering').value = 'floyd-steinberg';
             document.getElementById('paletteMode').value = 'auto';
 
             document.getElementById('fileInput').value = '';