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/index.html

@@ -81,7 +81,10 @@
 
         <div class="right-col">
           <div class="preview-panel">
-            <p class="eyebrow">Word 預覽</p>
+            <div class="preview-header">
+              <p class="eyebrow">Word 預覽</p>
+              <a id="previewOpenBtn" class="secondary-button preview-open-btn" target="_blank" hidden>開新分頁</a>
+            </div>
             <div class="preview-wrap">
               <iframe id="previewFrame" src="about:blank" title="Word 範本預覽"></iframe>
               <div id="previewLoading" class="preview-loading" hidden>