fix: prevent preview infinite loop and add document type filtering

- Remove onAutoConfigReceived callback that caused state update loop
- Add document analysis to check if file needs OCR track
- Only show preprocessing options for OCR-eligible files (images, scanned PDFs)
- Show informative message for editable PDFs that use direct text extraction
- Display text coverage percentage for editable documents

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/PreprocessingPreview.tsx

@@ -16,7 +16,6 @@ interface PreprocessingPreviewProps {
   config: PreprocessingConfig
   page?: number
   className?: string
-  onAutoConfigReceived?: (config: PreprocessingConfig) => void
 }
 
 export default function PreprocessingPreview({
@@ -25,7 +24,6 @@ export default function PreprocessingPreview({
   config,
   page = 1,
   className,
-  onAutoConfigReceived,
 }: PreprocessingPreviewProps) {
   const { t } = useTranslation()
   const [isLoading, setIsLoading] = useState(false)
@@ -81,18 +79,13 @@ export default function PreprocessingPreview({
 
       setOriginalImageUrl(URL.createObjectURL(originalBlob))
       setPreprocessedImageUrl(URL.createObjectURL(preprocessedBlob))
-
-      // Pass auto config to parent if available
-      if (response.auto_config && onAutoConfigReceived) {
-        onAutoConfigReceived(response.auto_config)
-      }
     } catch (err: any) {
       console.error('Preview fetch error:', err)
       setError(err.response?.data?.detail || err.message || 'Failed to load preview')
     } finally {
       setIsLoading(false)
     }
-  }, [taskId, debouncedMode, debouncedConfig, page, onAutoConfigReceived])
+  }, [taskId, debouncedMode, debouncedConfig, page])
 
   useEffect(() => {
     fetchPreview()