feat: Add 'New Scene' button to file list menu for creating linked planning+draft files

- Added create_scene_pair() method to workspace.rs
- Added '＋ New Scene' button in file_list.rs UI
- Button creates both planning file (05-Plot/act_N/scene_XX.md) and draft file (06-First Draft/Act N/sceneXX.md)
- Files are automatically linked via YAML frontmatter (draft_file and planning_template fields)
- Auto-detects current act from folder context
- Auto-generates next scene number
- Selects the draft file after creation (ready to edit)
- Includes hover text explaining the feature

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/app/file_list.rs

@@ -539,6 +539,16 @@ impl App {
                     if ui.button("＋ New").clicked() {
                         self.create_file();
                     }
+                    if ui
+                        .button("＋ New Scene")
+                        .on_hover_text(
+                            "Create a planning file in 05-Plot and a linked draft file \
+                             in 06-First Draft with automatic cross-references",
+                        )
+                        .clicked()
+                    {
+                        self.create_scene_pair();
+                    }
                 });
                 if ui
                     .button("＋ New from template")