feat: add preprocessing UI components and integration

Frontend implementation for add-layout-preprocessing proposal:
- Add PreprocessingSettings component with mode selection (auto/manual/disabled)
- Add manual config panel (contrast, sharpen, binarize options)
- Add zh-TW translations for preprocessing UI
- Integrate with ProcessingPage task start flow
- Add preprocessing types to apiV2.ts (PreprocessingMode, PreprocessingConfig)
- Pass preprocessing options to task start API

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

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

frontend/src/pages/ProcessingPage.tsx

@@ -10,9 +10,10 @@ import { useToast } from '@/components/ui/toast'
 import { apiClientV2 } from '@/services/apiV2'
 import { Play, CheckCircle, FileText, AlertCircle, Clock, Activity, Loader2 } from 'lucide-react'
 import LayoutModelSelector from '@/components/LayoutModelSelector'
+import PreprocessingSettings from '@/components/PreprocessingSettings'
 import TaskNotFound from '@/components/TaskNotFound'
 import { useTaskValidation } from '@/hooks/useTaskValidation'
-import type { LayoutModel, ProcessingOptions } from '@/types/apiV2'
+import type { LayoutModel, ProcessingOptions, PreprocessingMode, PreprocessingConfig } from '@/types/apiV2'
 
 export default function ProcessingPage() {
   const { t } = useTranslation()
@@ -34,6 +35,14 @@ export default function ProcessingPage() {
   // Layout model state (default to 'chinese' for best Chinese document support)
   const [layoutModel, setLayoutModel] = useState<LayoutModel>('chinese')
 
+  // Preprocessing state
+  const [preprocessingMode, setPreprocessingMode] = useState<PreprocessingMode>('auto')
+  const [preprocessingConfig, setPreprocessingConfig] = useState<PreprocessingConfig>({
+    contrast: 'clahe',
+    sharpen: true,
+    binarize: false,
+  })
+
   // Start OCR processing
   const processOCRMutation = useMutation({
     mutationFn: () => {
@@ -41,6 +50,8 @@ export default function ProcessingPage() {
         use_dual_track: true,
         language: 'ch',
         layout_model: layoutModel,
+        preprocessing_mode: preprocessingMode,
+        preprocessing_config: preprocessingMode === 'manual' ? preprocessingConfig : undefined,
       }
 
       return apiClientV2.startTask(taskId!, options)
@@ -342,13 +353,25 @@ export default function ProcessingPage() {
         </Card>
       )}
 
-      {/* Layout Model Selection (only show when task is pending) */}
+      {/* Processing Options (only show when task is pending) */}
       {isPending && (
-        <LayoutModelSelector
-          value={layoutModel}
-          onChange={setLayoutModel}
-          disabled={processOCRMutation.isPending}
-        />
+        <div className="space-y-6">
+          {/* Layout Model Selection */}
+          <LayoutModelSelector
+            value={layoutModel}
+            onChange={setLayoutModel}
+            disabled={processOCRMutation.isPending}
+          />
+
+          {/* Preprocessing Settings */}
+          <PreprocessingSettings
+            mode={preprocessingMode}
+            config={preprocessingConfig}
+            onModeChange={setPreprocessingMode}
+            onConfigChange={setPreprocessingConfig}
+            disabled={processOCRMutation.isPending}
+          />
+        </div>
       )}
     </div>
   )