Render chat and previews through the shared HTML cleaner

Why: The conversation and office fallbacks wrote marked output straight into the page, including converted docx HTML.
Evidence: html-sinks ratchet forbids new innerHTML writes outside @ui safeHtml.
Impact: Conversation keeps line-break rendering. Unsafe links are stripped after docx and pptx preview. dompurify is deduped with the UI package.
Refs: feature/sillytavern-review
Co-authored-by: Cursor <cursoragent@cursor.com>

PROJECT.md

@@ -10,4 +10,4 @@
 - `npx tsc -b` — 类型检查；`npm run build` — 产物 dist/（Dev Makefile 拷贝至 backend web/dist 内嵌）。
 - `npm test` — vitest：契约形状测试（Core 黄金样本驱动的判别联合/事件信封/HITL impact/工具三清单断言）。
 - 开发：`npm run dev`（Vite 代理 /api → 127.0.0.1:8787）。
-- 依赖刻意最小：react + marked；无 UI 框架（设计令牌手写，见 styles.css）。
+- 依赖刻意最小：react + marked + dompurify（模型输出与文档预览先清洗再写入页面）；无 UI 框架（设计令牌手写，见 styles.css）。