feat(memoir+conversation): 章节/故事最小可读字数；会话 hasUserMessage 与 UI 优化

- 后端：300 字门槛统一物化、hydrate、列表/PDF/详情；过短章节对读者隐藏
- 对话：首包前打字动画、大字模式排版、朗读/TTS 交互与布局稳定
- 首页：复用无用户消息会话；空列表「继续对话」与文案 i18n
- 章节阅读：标题进正文、封面与去重标题；阅读 Markdown 字号上调

api/app/features/memoir/chapter_markdown_compose.py

@@ -8,6 +8,9 @@ PDF 导出可单独物化「## 标题 + 正文」版本。
 from typing import Any
 
 from app.features.memoir.markdown_sanitize import sanitize_story_for_chapter_compose
+from app.features.memoir.reading_segment_materialize import (
+    story_meets_minimum_chapter_length,
+)
 
 
 def _gather_title_body_pairs(chapter: Any) -> list[tuple[str, str]]:
@@ -37,7 +40,7 @@ def compose_ordered_stories_to_markdown(ordered: list[tuple[str, str]]) -> str:
         if not raw:
             continue
         cleaned = sanitize_story_for_chapter_compose(raw, title)
-        if cleaned:
+        if cleaned and story_meets_minimum_chapter_length(cleaned):
             bodies.append(cleaned)
     return "\n\n---\n\n".join(bodies)
 
@@ -53,6 +56,8 @@ def compose_ordered_stories_to_pdf_markdown(ordered: list[tuple[str, str]]) -> s
         body = sanitize_story_for_chapter_compose(raw, title)
         if not body:
             continue
+        if not story_meets_minimum_chapter_length(body):
+            continue
         parts.append(f"## {t}\n\n{body}")
     return "\n\n---\n\n".join(parts)