style: move next chapter button to the left side toolbar

src/components/ViewerToolbar.tsx

@@ -57,6 +57,17 @@ export function ViewerToolbar({
           </svg>
           이전화
         </button>
+        <button
+          className="toolbar-btn chapter-nav-btn"
+          title="다음화"
+          disabled={!hasNext}
+          onClick={(e) => { e.stopPropagation(); onNextChapter(); }}
+        >
+          다음화
+          <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.5">
+            <path d="M5 12h14M12 5l7 7-7 7"/>
+          </svg>
+        </button>
       </div>
 
       {/* Center: Title + page info */}
@@ -110,17 +121,7 @@ export function ViewerToolbar({
           </button>
         </div>
 
-        <button
+
-          className="toolbar-btn chapter-nav-btn"
-          title="다음화"
-          disabled={!hasNext}
-          onClick={(e) => { e.stopPropagation(); onNextChapter(); }}
-        >
-          다음화
-          <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.5">
-            <path d="M5 12h14M12 5l7 7-7 7"/>
-          </svg>
-        </button>
 
         {/* Settings */}
         <button className="toolbar-btn" title="설정 (S)" onClick={(e) => { e.stopPropagation(); onToggleSettings(); }}>