feat: add translated PDF format selection (layout/reflow)

- Add generate_translated_layout_pdf() method for layout-preserving translated PDFs
- Add generate_translated_pdf() method for reflow translated PDFs
- Update translate router to accept format parameter (layout/reflow)
- Update frontend with dropdown to select translated PDF format
- Fix reflow PDF table cell extraction from content dict
- Add embedded images handling in reflow PDF tables
- Archive improve-translated-text-fitting openspec proposal

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

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

openspec/changes/improve-translated-text-fitting/proposal.md

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+# Change: Reflow Layout PDF Export for All Tracks
+
+## Why
+
+When generating translated PDFs, text often doesn't fit within original bounding boxes due to language expansion/contraction differences. Additionally, users may want a readable flowing document format even without translation.
+
+**Example from task c79df0ad-f9a6-4c04-8139-13eaef25fa83:**
+- Original Chinese: "华天科技（宝鸡）有限公司设备版块报价单" (19 characters)
+- Translated English: "Huatian Technology (Baoji) Co., Ltd. Equipment Division Quotation" (65+ characters)
+- Same bounding box: 703×111 pixels
+- Current result: Font reduced to minimum (3pt), text unreadable
+
+## What Changes
+
+- **NEW**: Add reflow layout PDF generation for both OCR and Direct tracks
+- Preserve semantic structure (headings, tables, lists) in reflow mode
+- Use consistent, readable font sizes (12pt body, 16pt headings)
+- Embed images inline within flowing content
+- **IMPORTANT**: Original layout preservation PDF generation remains unchanged
+- Support both tracks with proper reading order:
+  - **OCR track**: Use existing `reading_order` array from PP-StructureV3
+  - **Direct track**: Use PyMuPDF's implicit order (with option for column detection)
+- **FIX**: Remove outdated MADLAD-400 references from frontend (now uses Dify cloud translation)
+
+## Download Options
+
+| Scenario | Layout PDF | Reflow PDF |
+|----------|------------|------------|
+| **Without Translation** | Available | Available (NEW) |
+| **With Translation** | - | Available (single option, unchanged) |
+
+## Impact
+
+- Affected specs: `specs/result-export/spec.md`
+- Affected code:
+  - `backend/app/services/pdf_generator_service.py` - add reflow generation method
+  - `backend/app/routers/tasks.py` - add reflow PDF download endpoint
+  - `backend/app/routers/translate.py` - use reflow mode for translated PDFs
+  - `frontend/src/pages/TaskDetailPage.tsx`:
+    - Add "Download Reflow PDF" button for original documents
+    - Remove MADLAD-400 badge and outdated description text