feat: simplify layout model selection and archive proposals

Changes:
- Replace PP-Structure 7-slider parameter UI with simple 3-option layout model selector
- Add layout model mapping: chinese (PP-DocLayout-S), default (PubLayNet), cdla
- Add LayoutModelSelector component and zh-TW translations
- Fix "default" model behavior with sentinel value for PubLayNet
- Add gap filling service for OCR track coverage improvement
- Add PP-Structure debug utilities
- Archive completed/incomplete proposals:
  - add-ocr-track-gap-filling (complete)
  - fix-ocr-track-table-rendering (incomplete)
  - simplify-ppstructure-model-selection (22/25 tasks)
- Add new layout model tests, archive old PP-Structure param tests
- Update OpenSpec ocr-processing spec with layout model requirements

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

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

frontend/src/components/LayoutModelSelector.tsx

@@ -0,0 +1,110 @@
+import { cn } from '@/lib/utils'
+import { Check, FileText, Globe, BookOpen } from 'lucide-react'
+import { useTranslation } from 'react-i18next'
+import type { LayoutModel } from '@/types/apiV2'
+
+interface LayoutModelSelectorProps {
+  value: LayoutModel
+  onChange: (model: LayoutModel) => void
+  disabled?: boolean
+  className?: string
+}
+
+const MODEL_ICONS: Record<LayoutModel, React.ReactNode> = {
+  chinese: <FileText className="w-5 h-5" />,
+  default: <Globe className="w-5 h-5" />,
+  cdla: <BookOpen className="w-5 h-5" />,
+}
+
+export default function LayoutModelSelector({
+  value,
+  onChange,
+  disabled = false,
+  className,
+}: LayoutModelSelectorProps) {
+  const { t } = useTranslation()
+  const models: LayoutModel[] = ['chinese', 'default', 'cdla']
+
+  const getModelInfo = (model: LayoutModel) => ({
+    label: t(`processing.layoutModel.${model}`),
+    description: t(`processing.layoutModel.${model}Desc`),
+  })
+
+  return (
+    <div className={cn('border rounded-lg p-4 bg-white', className)}>
+      {/* Header */}
+      <div className="flex items-center gap-2 mb-4">
+        <FileText className="w-5 h-5 text-gray-600" />
+        <h3 className="text-lg font-semibold text-gray-900">{t('processing.layoutModel.title')}</h3>
+      </div>
+
+      {/* Model Options */}
+      <div className="space-y-3">
+        {models.map((model) => {
+          const info = getModelInfo(model)
+          const isSelected = value === model
+
+          return (
+            <button
+              key={model}
+              type="button"
+              disabled={disabled}
+              onClick={() => onChange(model)}
+              className={cn(
+                'w-full flex items-start gap-4 p-4 rounded-lg border-2 transition-all text-left',
+                isSelected
+                  ? 'border-blue-500 bg-blue-50'
+                  : 'border-gray-200 hover:border-gray-300 hover:bg-gray-50',
+                disabled && 'opacity-50 cursor-not-allowed'
+              )}
+            >
+              {/* Icon */}
+              <div
+                className={cn(
+                  'p-2 rounded-lg flex-shrink-0',
+                  isSelected ? 'bg-blue-100 text-blue-600' : 'bg-gray-100 text-gray-500'
+                )}
+              >
+                {MODEL_ICONS[model]}
+              </div>
+
+              {/* Content */}
+              <div className="flex-1 min-w-0">
+                <div className="flex items-center gap-2">
+                  <span
+                    className={cn(
+                      'font-medium',
+                      isSelected ? 'text-blue-700' : 'text-gray-900'
+                    )}
+                  >
+                    {info.label}
+                  </span>
+                  {model === 'chinese' && (
+                    <span className="text-xs bg-green-100 text-green-700 px-2 py-0.5 rounded-full">
+                      {t('processing.layoutModel.recommended')}
+                    </span>
+                  )}
+                </div>
+                <p className="text-sm text-gray-500 mt-1">{info.description}</p>
+              </div>
+
+              {/* Check mark */}
+              {isSelected && (
+                <div className="flex-shrink-0">
+                  <Check className="w-5 h-5 text-blue-600" />
+                </div>
+              )}
+            </button>
+          )
+        })}
+      </div>
+
+      {/* Info Note */}
+      <div className="mt-4 p-3 bg-blue-50 border border-blue-200 rounded-md">
+        <p className="text-sm text-blue-800">
+          {t('processing.layoutModel.note')}
+        </p>
+      </div>
+    </div>
+  )
+}