fix: 修復PDF生成中的頁碼錯誤和文字重疊問題

## 問題修復

### 1. 頁碼分配錯誤
- **問題**: layout_data 和 images_metadata 頁碼被 1-based 覆蓋，導致全部為 0
- **修復**: 在 analyze_layout() 添加 current_page 參數，從源頭設置正確的 0-based 頁碼
- **影響**: 表格和圖片現在顯示在正確的頁面上

### 2. 文字與表格/圖片重疊
- **問題**: 使用不存在的 'tables' 和 'image_regions' 字段過濾，導致過濾失效
- **修復**: 改用 images_metadata（包含所有表格/圖片的 bbox）
- **新增**: _bbox_overlaps() 檢測任意重疊（非完全包含）
- **影響**: 文字不再覆蓋表格和圖片區域

### 3. 渲染順序優化
- **調整**: 圖片(底層) → 表格(中間層) → 文字(頂層)
- **影響**: 視覺層次更正確

## 技術細節

- ocr_service.py: 添加 current_page 參數傳遞，移除頁碼覆蓋邏輯
- pdf_generator_service.py:
  - 新增 _bbox_overlaps() 方法
  - 更新 _filter_text_in_regions() 使用重疊檢測
  - 修正數據源為 images_metadata
  - 調整繪製順序

## 已知限制

- 仍有 21.6% 文字因過濾而遺失（座標定位方法的固有問題）
- 未使用 PP-StructureV3 的完整版面資訊（parsing_res_list, layout_bbox）

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

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

openspec/changes/fix-result-preview-and-pdf-download/specs/task-management/spec.md

@@ -0,0 +1,63 @@
+# Task Management - Delta Changes
+
+## MODIFIED Requirements
+
+### Requirement: Task Result Display
+The system SHALL provide interactive PDF preview of OCR results with layout preservation on Results and Task Detail pages.
+
+#### Scenario: Results page shows layout-preserving PDF preview
+- **WHEN** Results page loads with a completed task
+- **THEN** page SHALL fetch PDF from `/api/v2/tasks/{task_id}/download/pdf`
+- **AND** page SHALL render PDF using react-pdf PDFViewer component
+- **AND** page SHALL NOT show placeholder text "請使用上方下載按鈕..."
+- **AND** PDF SHALL display with original document layout preserved
+- **AND** PDF SHALL support zoom and page navigation controls
+
+#### Scenario: Task detail page shows PDF preview
+- **WHEN** Task Detail page loads for a completed task
+- **THEN** page SHALL fetch layout-preserving PDF
+- **AND** page SHALL render PDF using PDFViewer component
+- **AND** page SHALL NOT show placeholder text
+- **AND** PDF SHALL visually match original document layout
+
+#### Scenario: Preview handles loading state
+- **WHEN** PDF is being generated or fetched
+- **THEN** page SHALL display loading spinner
+- **AND** page SHALL show progress indicator during PDF generation
+- **AND** page SHALL NOT show error or placeholder text
+
+#### Scenario: Preview handles errors gracefully
+- **WHEN** PDF generation fails or file is missing
+- **THEN** page SHALL display helpful error message
+- **AND** error message SHALL suggest trying download again or contact support
+- **AND** page SHALL NOT crash or expose technical errors to user
+- **AND** page MAY fallback to markdown preview if PDF unavailable
+
+## ADDED Requirements
+
+### Requirement: Interactive PDF Viewer Features
+The PDF viewer component SHALL provide essential viewing controls for user convenience.
+
+#### Scenario: PDF viewer provides zoom controls
+- **WHEN** user views PDF preview
+- **THEN** viewer SHALL provide zoom in (+) and zoom out (-) buttons
+- **AND** viewer SHALL provide fit-to-width option
+- **AND** viewer SHALL provide fit-to-page option
+- **AND** zoom level SHALL persist during page navigation
+
+#### Scenario: PDF viewer provides page navigation
+- **WHEN** PDF contains multiple pages
+- **THEN** viewer SHALL display current page number and total pages
+- **AND** viewer SHALL provide previous/next page buttons
+- **AND** viewer SHALL provide page selector dropdown
+- **AND** page navigation SHALL be smooth without flickering
+
+### Requirement: Frontend PDF Library Integration
+The frontend SHALL use react-pdf for PDF rendering capabilities.
+
+#### Scenario: react-pdf configured correctly
+- **WHEN** application initializes
+- **THEN** react-pdf library SHALL be installed and imported
+- **AND** PDF.js worker SHALL be configured properly
+- **AND** worker path SHALL point to correct pdfjs-dist worker file
+- **AND** PDF rendering SHALL work without console errors