feat: Replace cookiecutter with built-in MiniJinja templating

- Add MiniJinja dependency for Jinja2-compatible templating
- Create new project_template.rs module for template rendering
- Bundle snowflake template in templates/snowflake/ directory
- Remove dependency on external cookiecutter Python tool
- Handle Gitea publishing natively in Rust instead of Python hooks
- Update default template path to use bundled template with fallback
- Update README to reflect new built-in templating
- All tests pass

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

src/config.rs

@@ -174,7 +174,14 @@ pub fn default_show_toolbar() -> bool {
 }
 
 /// Default cookiecutter template for **File ▸ New project…**.
+/// First tries the bundled template, then falls back to the user's Cookiecutters directory.
 pub fn default_project_template() -> PathBuf {
+    // Try the bundled template first
+    let bundled = PathBuf::from("templates/snowflake");
+    if bundled.join("cookiecutter.json").is_file() {
+        return bundled;
+    }
+    // Fallback to user's Cookiecutters directory
     dirs::home_dir()
         .unwrap_or_else(|| PathBuf::from("."))
         .join("Documents")