Add template download UI to dashboard

[sarkarshrayan2-max]

Parent issue

Part of #246.

Dependency

Blocked by #258.

Description

Add a dashboard interface for downloading Fenn templates without requiring the user to use the CLI.

Requirements

• fetch available templates from GET /api/templates
• display templates in a dropdown or selectable list
• allow the user to enter a target directory
• allow the user to enable force overwrite
• call POST /api/templates/pull
• display a loading or progress state during download
• display a clear success result
• display structured API errors
• disable repeated submissions while downloading
• preserve authentication and CSRF protection

Acceptance criteria

• a user can select an available template
• a user can enter a destination directory
• a user can enable or disable force overwrite
• the template can be downloaded from the dashboard
• successful downloads are clearly displayed
• failed downloads show useful error messages
• tests cover success, validation errors, and API failures