feat: 品質モードのデフォルトを最高品質に変更

- 空間的量子化による最高品質をデフォルトに
- エッジや細部を最も良く保持
- リセット時も最高品質に戻るように設定

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

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

Author: ayutaz

index.html

@@ -51,9 +51,9 @@ <h2 class="section-title">変換設定</h2>
                         <div class="setting-control">
                             <select id="qualityMode" class="select">
                                 <option value="fast">高速（単純量子化）</option>
-                                <option value="standard" selected>標準（K-means）</option>
+                                <option value="standard">標準（K-means）</option>
                                 <option value="high">高品質（Median Cut）</option>
-                                <option value="best">最高品質（空間的量子化）</option>
+                                <option value="best" selected>最高品質（空間的量子化）</option>
                             </select>
                         </div>
                     </div>

src/js/app-v3.js

@@ -729,7 +729,7 @@
             const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
 
             // Select algorithm based on quality mode
-            const qualityMode = document.getElementById('qualityMode')?.value || 'standard';
+            const qualityMode = document.getElementById('qualityMode')?.value || 'best';
             let palette;
 
             switch(qualityMode) {
@@ -1347,6 +1347,11 @@
             document.getElementById('dithering').value = 'floyd-steinberg';
             document.getElementById('paletteMode').value = 'auto';
 
+            const qualityMode = document.getElementById('qualityMode');
+            if (qualityMode) {
+                qualityMode.value = 'best';
+            }
+            
             document.getElementById('fileInput').value = '';
 
             const sections = ['fileListSection', 'settingsSection', 'previewSection', 'downloadSection'];