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>

openspec/changes/archive/2025-11-26-fix-ocr-table-empty-columns/proposal.md

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+# Change: Fix OCR Track Table Empty Columns and Alignment
+
+## Why
+
+PP-Structure 生成的表格經常包含空白欄位（所有 row 該欄皆為空/空白），導致轉換後的 UnifiedDocument 表格出現空欄與欄位錯位。目前 OCR Track 直接使用原始資料，未進行清理，影響 PDF/JSON/Markdown 輸出品質。
+
+## What Changes
+
+- 新增 `trim_empty_columns()` 函數，清理 OCR Track 表格的空欄
+- 在 `_convert_table_data` 入口調用清洗邏輯，確保 TableData 乾淨
+- 處理 col_span 重算：若 span 跨過被移除欄位，縮小 span
+- 更新 columns/cols 數值、調整各 cell 的 col 索引
+- 可選：依 bbox x0 進行欄對齊排序
+
+## Impact
+
+- Affected specs: `ocr-processing`
+- Affected code:
+  - `backend/app/services/ocr_to_unified_converter.py` (主要修改)
+- 不影響 Direct/HYBRID 路徑
+- PDF/JSON/Markdown 輸出將更乾淨
+
+## Constraints
+
+- 保持表格 bbox、頁面座標不變
+- 不修改 Direct/HYBRID 路徑
+- 只移除「所有行皆空」的欄；若表頭空但數據有值，不應移除
+- 保留原 bbox，避免 PDF 版面漂移