feat(djinit/utils): add replace_app_in_user_defined_apps and extend startapp with custom app_module support

This change introduces a helper to replace app entries in USER_DEFINED_APPS and makes the startapp command more flexible by allowing callers to specify the app module name.

New Features:
- Add replace_app_in_user_defined_apps utility function in djinit/utils/common.py
- Introduce app_config_name context and accept app_module argument in DjangoHelper.startapp()

Enhancements:
- Preserve indentation when replacing entries
- Allow custom app module specification instead of defaulting to app_name

Bug Fixes: (none)

djinit/utils/common.py

@@ -203,6 +203,33 @@ def insert_apps_into_user_defined_apps(content: str, apps_to_add: list) -> str:
     return "\n".join(new_lines)
 
 
+def replace_app_in_user_defined_apps(content: str, old_app: str, new_app: str) -> str | None:
+    """Replace an app entry in USER_DEFINED_APPS with a new one."""
+    lines = content.split("\n")
+    new_lines = []
+    replaced = False
+
+    for line in lines:
+        if not replaced and _parse_app_entry_from_line(line) == old_app:
+            # Try to preserve indentation
+            indent_idx = 0
+            for char in line:
+                if char.isspace():
+                    indent_idx += 1
+                else:
+                    break
+            indent = line[:indent_idx] if indent_idx > 0 else "    "
+            
+            new_lines.append(f'{indent}"{new_app}",')
+            replaced = True
+        else:
+            new_lines.append(line)
+
+    if replaced:
+        return "\n".join(new_lines)
+    return None
+
+
 def calculate_app_module_paths(app_names: list, metadata: dict) -> list:
     nested = bool(metadata.get("nested_apps"))
     nested_dir_name = metadata.get("nested_dir") if nested else None

djinit/utils/django.py

@@ -60,12 +60,24 @@ class DjangoHelper:
             raise DjinitError(f"Error creating Django project: {e}", details=str(e)) from e
 
     @staticmethod
-    def startapp(app_name: str, directory: str) -> None:
+    def startapp(app_name: str, directory: str, app_module: str = None) -> None:
         try:
             app_dir = os.path.join(directory, app_name)
             os.makedirs(app_dir, exist_ok=True)
 
-            context = {"app_name": app_name, "django_version": DjangoHelper.DJANGO_VERSION}
+            # Prepare context
+            app_config_name = app_name.title().replace('_', '')
+            context = {
+                "app_name": app_name,
+                "app_config_name": app_config_name,
+                "django_version": DjangoHelper.DJANGO_VERSION
+            }
+            
+            # Use provided app_module or default to app_name (path calculation handled by caller)
+            if not app_module:
+                app_module = app_name
+                
+            context["app_module"] = app_module
 
             create_init_file(app_dir, f"Created {app_name}/__init__.py")