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/i18n/locales/zh-TW.json

@@ -52,6 +52,17 @@
       "language": "識別語言",
       "threshold": "信心度閾值",
       "layoutDetection": "版面偵測"
+    },
+    "layoutModel": {
+      "title": "版面偵測模型",
+      "chinese": "中文文件模型",
+      "chineseDesc": "PP-DocLayout-S - 適用於中文表單、合約、發票（推薦）",
+      "default": "標準模型",
+      "defaultDesc": "PubLayNet 模型 - 適用於英文學術論文、報告",
+      "cdla": "CDLA 模型",
+      "cdlaDesc": "專用中文版面分析模型 - 適用於複雜中文版面",
+      "recommended": "推薦",
+      "note": "版面模型會影響文件結構（表格、文字區塊、圖片）的偵測效果。請根據您的文件類型選擇適合的模型。"
     }
   },
   "results": {