Import Feature to Improve Onboarding

[perber]

To improve the onboarding experience, an import feature should be introduced.

An admin user can upload a ZIP file containing Markdown content via the UI.
After upload, the system analyzes the content and adapts it to the Leafwikis filesystem structure.

Before executing the import, the user is presented with a preview of all detected content and changes, including:

• Files to be imported
• Folder and path changes required for the Leafwikis filesystem organization
• New vs. existing files

The user can then explicitly decide which content should be imported.

Acceptance Criteria

• An admin user can upload a ZIP file containing Markdown files via the UI
• After upload, a preview is shown that includes:
  • A list of all detected files
  • The required changes to align with the Leafwikis filesystem structure
• The user can confirm which files and folders should be imported
• The import can be executed even if data already exists in the system
• Existing data is not overwritten without explicit confirmation (controlled conflict handling)