fix: Add effective_project_template() to fallback to bundled template

- Add effective_project_template() method that checks if saved template path exists
- If not, falls back to default_project_template() (bundled template)
- This handles users who have old config.json pointing to deleted Cookiecutters path
- Update project.rs to use effective_project_template() when creating new projects

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

src/app/project.rs

@@ -61,7 +61,7 @@ impl App {
             self.np_status = format!("{} already exists.", parent.join(&name).display());
             return;
         }
-        let template = self.config.project_template.clone();
+        let template = self.config.effective_project_template();
         
         // Check that the template directory exists
         if !template.is_dir() {
@@ -600,8 +600,8 @@ impl App {
 
         if browse_template {
             let mut dialog = rfd::FileDialog::new().set_title("Choose a cookiecutter template");
-            if self.config.project_template.is_dir() {
-                dialog = dialog.set_directory(&self.config.project_template);
+            if self.config.effective_project_template().is_dir() {
+                dialog = dialog.set_directory(&self.config.effective_project_template());
             }
             if let Some(path) = dialog.pick_folder() {
                 self.config.project_template = path;