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/components/content_input.py

@@ -65,6 +65,7 @@ def render_content_input():
                     "paragraph": tr("split.mode_paragraph"),
                     "line": tr("split.mode_line"),
                     "sentence": tr("split.mode_sentence"),
+                    "smart": tr("split.mode_smart"),
                 }
                 split_mode = st.selectbox(
                     tr("split.mode_label"),
@@ -73,8 +74,16 @@ def render_content_input():
                     index=0,  # Default to paragraph mode
                     help=tr("split.mode_help")
                 )
+                
+                # Show info for smart mode (auto-detect segment count)
+                if split_mode == "smart":
+                    st.info(tr("split.smart_auto_hint"))
+                    target_segments = None  # Auto-detect
+                else:
+                    target_segments = None  # Not used for other modes
             else:
                 split_mode = "paragraph"  # Default for generate mode (not used)
+                target_segments = None
             
             # Title input (optional for both modes)
             title = st.text_input(
@@ -105,7 +114,8 @@ def render_content_input():
                 "text": text,
                 "title": title,
                 "n_scenes": n_scenes,
-                "split_mode": split_mode
+                "split_mode": split_mode,
+                "target_segments": target_segments
             }
         
         else: