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。

src/main/document-parsing-service.test.ts

@@ -3,6 +3,7 @@ import {
   defaultDocumentParsingSettings
 } from './document-parsing-settings-store'
 import { DocumentParsingService } from './document-parsing-service'
+import { createImagePptx } from '../../tests/support/pptx-fixture'
 
 function createPdfFixture(...pageTexts: string[]): Buffer {
   const texts = pageTexts.length > 0 ? pageTexts : ['']
@@ -109,6 +110,73 @@ function createService(overrides?: {
 }
 
 describe('DocumentParsingService', () => {
+  it.each(['chat-attachment', 'knowledge-index', 'artifact-import', 'diagnostic'] as const)(
+    'recognizes embedded PPTX images for %s',
+    async (purpose) => {
+      const { recognize, service } = createService()
+      const parsed = await service.parse('slides.pptx', createImagePptx('', 2), purpose)
+      expect(parsed.sourceFormat).toBe('.pptx')
+      expect(parsed.pageCount).toBe(2)
+      expect(parsed.sections.map(({ locator, method, pageNumber }) => ({
+        locator, method, pageNumber
+      }))).toEqual([
+        { locator: '幻灯片 1 · 图片 1', method: 'ocr', pageNumber: 1 },
+        { locator: '幻灯片 2 · 图片 1', method: 'ocr', pageNumber: 2 }
+      ])
+      expect(recognize).toHaveBeenCalledTimes(2)
+      expect(recognize).toHaveBeenCalledWith(expect.objectContaining({
+        mimeType: 'image/png', maximumPages: 1, modelId: 'pp-ocrv6-tiny'
+      }))
+    }
+  )
+
+  it('preserves PPTX native text while recognizing image content on the same slide', async () => {
+    const { service } = createService()
+    const parsed = await service.parse('slides.pptx', createImagePptx('Native slide title'), 'chat-attachment')
+    expect(parsed.content).toContain('Native slide title')
+    expect(parsed.content).toContain('扫描件识别正文')
+    const diagnostic = await service.diagnose('slides.pptx', createImagePptx('Native slide title'))
+    expect(diagnostic).toMatchObject({ method: 'mixed', pageCount: 1, ocrPageCount: 1 })
+  })
+
+  it('reports disabled OCR for image-only PPTX in fast-text mode', async () => {
+    const { service, recognize } = createService({ settings: { chatWorkflow: 'fast-text' } })
+    await expect(service.parse('slides.pptx', createImagePptx(), 'chat-attachment')).rejects.toThrow('当前工作流未启用 OCR')
+    expect(recognize).not.toHaveBeenCalled()
+  })
+
+  it('enforces PPTX OCR page limits before recognizing images', async () => {
+    const { service, recognize } = createService({ settings: { maximumPages: 1 } })
+    await expect(service.parse('slides.pptx', createImagePptx('', 2), 'knowledge-index')).rejects.toThrow('超过 1 页限制')
+    expect(recognize).not.toHaveBeenCalled()
+  })
+
+  it('reports the model error for image-only PPTX, not empty document text', async () => {
+    const { service } = createService({ modelStatus: {
+      available: false, verified: false, detail: '模型尚未安装'
+    } })
+    await expect(service.parse('slides.pptx', createImagePptx(), 'chat-attachment')).rejects.toThrow('模型尚未安装')
+  })
+
+  it('only falls back to PPTX native text for automatic non-index workflows', async () => {
+    const { service } = createService({ recognize: async () => { throw new Error('OCR runtime failed') } })
+    const bytes = createImagePptx('Useful native slide body')
+    expect((await service.parse('slides.pptx', bytes, 'chat-attachment')).warnings).toEqual([
+      expect.stringContaining('OCR runtime failed')
+    ])
+    await expect(service.parse('slides.pptx', bytes, 'knowledge-index')).rejects.toThrow('OCR runtime failed')
+  })
+
+  it('stops PPTX OCR between images on cancellation instead of returning native text', async () => {
+    const controller = new AbortController()
+    const { service, recognize } = createService({ recognize: async () => {
+      controller.abort(new Error('Cancelled PPTX'))
+      throw new Error('OCR stopped')
+    } })
+    await expect(service.parse('slides.pptx', createImagePptx('Useful native text', 2), 'chat-attachment', controller.signal)).rejects.toThrow('Cancelled PPTX')
+    expect(recognize).toHaveBeenCalledOnce()
+  })
+
   it('keeps useful PDF text local without invoking OCR', async () => {
     const { recognize, service } = createService()