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/service.py

@@ -28,6 +28,9 @@ from app.features.memoir.helpers import (
 )
 from app.features.memoir.memoir_images.settings import MemoirImageSettings
 from app.features.memoir.models import Book, Chapter, ChapterStoryLink
+from app.features.memoir.reading_segment_materialize import (
+    chapter_meets_minimum_display,
+)
 from app.features.memory.service import MemoryService
 from app.ports.storage import ObjectStorage
 
@@ -144,7 +147,12 @@ class MemoirService:
             .order_by(Chapter.order_index)
         )
         result = await self._db.execute(stmt)
-        chapters = list(result.unique().scalars().all())
+        chapters_raw = list(result.unique().scalars().all())
+        chapters: List[Chapter] = []
+        for ch in chapters_raw:
+            ch2 = await self._ensure_chapter_materialized(ch)
+            if chapter_meets_minimum_display(ch2):
+                chapters.append(ch2)
         asset_ids = collect_asset_ids_for_chapters(chapters)
         asset_map = await signed_urls_for_asset_ids(self._db, asset_ids)
         pdf_bytes = await pdf_service.generate_pdf(
@@ -170,6 +178,9 @@ class MemoirService:
         asset_map = await signed_urls_for_asset_ids(self._db, asset_ids)
         all_chapters: List[dict] = []
         for ch in chapters:
+            ch = await self._ensure_chapter_materialized(ch)
+            if not chapter_meets_minimum_display(ch):
+                continue
             await self._cleanup_unavailable_images(ch)
             all_chapters.append(chapter_to_list_dict(ch, asset_url_map=asset_map))
         return all_chapters
@@ -184,6 +195,8 @@ class MemoirService:
             raise HTTPException(status_code=404, detail="Chapter not found")
         chapter = await self._ensure_chapter_materialized(chapter)
         await self._cleanup_unavailable_images(chapter)
+        if not chapter_meets_minimum_display(chapter):
+            raise HTTPException(status_code=404, detail="Chapter not found")
         asset_map = await signed_urls_for_asset_ids(
             self._db, collect_asset_ids_for_chapter(chapter)
         )