feat(typesetting): 段落按句拆分填满页面剩余空间 - 减少空白浪费

- paginateContent正常块换页判断增加按句拆分逻辑
- p/blockquote超过剩余空间时调用splitOversizedBlock拆分
- 拆分后的句子块逐步填入当前页，多余推到后续页
- img/pre/table/h2/h3/h4不拆分，直接换页
- remainingSpace>50条件防止极小碎片
- 拆分失败时回退到原有换页逻辑

public/static/js/phone-image.js

@@ -663,6 +663,33 @@ var PhoneImageEngine = (function () {
             var nextBlock = (i + 1 < blocks.length) ? blocks[i + 1] : null;
 
             if (currentHeight + block.estimatedHeight > contentAreaHeight && currentPageBlocks.length > 0) {
+                var remainingSpace = contentAreaHeight - currentHeight;
+                // 检查是否可以按句拆分来填满剩余空间
+                var canSplit = (block.type === 'p' || block.type === 'blockquote') &&
+                    block.estimatedHeight <= contentAreaHeight && remainingSpace > 50;
+
+                if (canSplit) {
+                    // 按句拆分，逐步填入页面
+                    var splitParts = splitOversizedBlock(block, contentAreaHeight);
+                    if (splitParts.length > 1) {
+                        // 逐句填入当前页剩余空间
+                        for (var sp = 0; sp < splitParts.length; sp++) {
+                            if (currentHeight + splitParts[sp].estimatedHeight > contentAreaHeight && currentPageBlocks.length > 0) {
+                                contentPages.push(generateContentPage(
+                                    currentPageBlocks, pageNumber, sizeConfig, false
+                                ));
+                                currentPageBlocks = [];
+                                currentHeight = 0;
+                                pageNumber++;
+                            }
+                            currentPageBlocks.push(splitParts[sp]);
+                            currentHeight += splitParts[sp].estimatedHeight;
+                        }
+                        // 跳过后面的 currentPageBlocks.push(block)，用拆分后的块代替
+                        continue;
+                    }
+                }
+                // 不可拆分(img/pre/table/h)或拆分失败，直接换页
                 contentPages.push(generateContentPage(
                     currentPageBlocks, pageNumber, sizeConfig, false
                 ));