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>

web/i18n/locales/en_US.json

@@ -26,6 +26,8 @@
     "split.mode_paragraph": "📄 By Paragraph (\\n\\n)",
     "split.mode_line": "📝 By Line (\\n)",
     "split.mode_sentence": "✂️ By Sentence (。.!?)",
+    "split.mode_smart": "🧠 Smart Merge (AI Grouping)",
+    "split.smart_auto_hint": "🤖 AI will analyze text structure, recommend optimal segment count, and intelligently merge related paragraphs (dialogues, same scene)",
     "input.content": "Content",
     "input.content_placeholder": "Used directly without modification (split by strategy below)\nExample:\nHello everyone, today I'll share three study tips.\n\nThe first tip is focus training, meditate for 10 minutes daily.\n\nThe second tip is active recall, review immediately after learning.",
     "input.content_help": "Provide your own content for video generation",