fix: recognize PPTX images and release idle OCR memory

Image-only PPTX attachments failed with no indexable text even when a local
OCR model was configured. PPTX parsing now recognizes embedded PNG, JPEG
and WebP images through the existing local OCR path, preserves native text
and slide locators, and honors the selected parsing workflow.

OCR workers previously retained model sessions and WASM memory indefinitely
after successful parsing. Release the worker after 60 seconds with no active
or queued requests, reuse it for continuous work, and reload on demand.
Fast text/index modes still skip OCR. This does not render Office layouts,
recognize inherited master images, or change Agent execution permissions.

Validated four real PPTX attachments (40 pages), idle memory release and a
nine-page reimport after model reload on Windows x64 with PP-OCRv6 Tiny.
49 local OCR requests and zero text-model calls. All 4293 tests passed
with 67 skipped; typecheck, lint and the isolated Electron build passed.

Release note: 修复已配置本地 OCR 时图片型 PPTX 仍无法导入的问题，识别结果保留幻灯片定位。OCR 空闲 60 秒后自动释放模型运行内存，连续解析复用模型，下次使用时自动重新加载。

docs/features/document-processing/README.md

@@ -7,7 +7,7 @@
 | 文档 | 权威职责 |
 | --- | --- |
 | [文档解析与本地 OCR PRD](./prd.md) | 产品范围、解析策略、OCR 模型管理和验收 |
-| [聊天附件技术说明](./chat-attachments-technical-design.md) | 文件选择与粘贴的进程链路、IPC、失败处理和验证方法 |
+| [聊天附件技术说明](./chat-attachments-technical-design.md) | 文件选择与粘贴的进程链路、IPC、PPTX 图片 OCR、Worker 生命周期和验证证据 |
 
 当前尚无独立 User Stories、功能逻辑、UI 和进度文档；聊天输入区交互遵循根目录
 [UI 设计系统](../../../UI-DESIGN.md)。