docs: Update all references from old 05-Plot/06-First Draft to merged 05-First Draft

- Update project_template test to verify old directories are NOT created
- Update hover text and comments in project.rs
- Update comments and code in file_list.rs
- Update comment in spelling.rs
- Add test assertions to verify 05-Plot and 06-First Draft don't exist in new projects

This ensures all code references match the merged directory structure where
planning and draft content are combined in 05-First Draft with YAML frontmatter.

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

src/project_template.rs

@@ -348,12 +348,23 @@ mod tests {
         assert!(readme_content.contains("MiniJinja Test Project"));
         assert!(readme_content.contains("A project generated with MiniJinja"));
 
-        // Check that the 06-First Draft subdirectory exists (default manuscript folder)
+        // Check that the 05-First Draft subdirectory exists (default manuscript folder)
         let subdir = crate::config::default_project_open_subdir();
         assert!(
             project.join(&subdir).is_dir(),
             "expected a {subdir} folder in {project:?}"
         );
+        
+        // Verify old directories (05-Plot, 06-First Draft) are NOT created
+        // (they were merged into 05-First Draft)
+        assert!(
+            !project.join("05-Plot").is_dir(),
+            "05-Plot should not exist (merged into 05-First Draft)"
+        );
+        assert!(
+            !project.join("06-First Draft").is_dir(),
+            "06-First Draft should not exist (merged into 05-First Draft)"
+        );
 
         // Clean up
         let _ = std::fs::remove_dir_all(&out);