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/app.js

@@ -8,6 +8,7 @@ const resultTitle = document.querySelector('#resultTitle');
 const connectionStatus = document.querySelector('#connectionStatus');
 const previewFrame = document.querySelector('#previewFrame');
 const previewLoading = document.querySelector('#previewLoading');
+const previewOpenBtn = document.querySelector('#previewOpenBtn');
 
 let currentPdfBlob = null;
 let currentPdfFileName = '租屋契約.pdf';
@@ -168,6 +169,8 @@ async function loadPreview() {
     if (previewBlobUrl) URL.revokeObjectURL(previewBlobUrl);
     previewBlobUrl = URL.createObjectURL(blob);
     previewFrame.src = `${previewBlobUrl}#toolbar=0`;
+    previewOpenBtn.href = previewBlobUrl;
+    previewOpenBtn.hidden = false;
   } catch {
     // AbortError 或其他錯誤都忽略，交給 finally 處理
   } finally {