Word 預覽：修正手機只能看第一頁、載入中遮罩無法關閉

- 預覽面板新增「開新分頁」按鈕，手機可完整瀏覽所有頁面
- 修正 [hidden] 被 CSS display:flex 覆蓋導致載入中遮罩卡住的問題
- 改用 generation counter 取代 AbortController signal 判斷載入狀態

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles.css

@@ -218,10 +218,26 @@ h2 {
   min-height: 400px;
 }
 
-.preview-panel .eyebrow {
+.preview-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 12px;
+}
+
+.preview-header .eyebrow {
   margin: 0;
 }
 
+.preview-open-btn {
+  min-height: 32px;
+  padding: 0 12px;
+  font-size: 0.85rem;
+  text-decoration: none;
+  display: inline-flex;
+  align-items: center;
+}
+
 .preview-wrap {
   flex: 1;
   position: relative;