feat: Add smart paragraph merging mode with AI grouping

- Add "smart" split mode that uses LLM to intelligently merge related paragraphs
- Implement two-step approach: analyze text structure, then group by semantic relevance
- Add paragraph_merging.py with analysis and grouping prompts
- Update UI to support smart mode selection with auto-detect hint
- Add i18n translations for smart mode (en_US, zh_CN)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pixelle_video/prompts/__init__.py

@@ -29,6 +29,13 @@ from pixelle_video.prompts.image_generation import (
 )
 from pixelle_video.prompts.style_conversion import build_style_conversion_prompt
 
+# Paragraph merging (two-step: analysis + grouping)
+from pixelle_video.prompts.paragraph_merging import (
+    build_paragraph_analysis_prompt,
+    build_paragraph_grouping_prompt,
+    build_paragraph_merging_prompt,  # Legacy support
+)
+
 
 __all__ = [
     # Narration builders
@@ -40,6 +47,11 @@ __all__ = [
     "build_image_prompt_prompt",
     "build_style_conversion_prompt",
     
+    # Paragraph merging (two-step)
+    "build_paragraph_analysis_prompt",
+    "build_paragraph_grouping_prompt",
+    "build_paragraph_merging_prompt",  # Legacy
+    
     # Image style presets
     "IMAGE_STYLE_PRESETS",
     "DEFAULT_IMAGE_STYLE",