refactor: unify model download system across all model types

- Add download-related methods to baseModelApi.js for fetching versions, roots, folders, and downloading models
- Replace separate download managers with a unified DownloadManager.js supporting all model types
- Create a single download_modals.html template that adapts to model type (LoRA, checkpoint, etc.)
- Remove old download modals from lora_modals.html and checkpoint_modals.html
- Update apiConfig.js to include civitaiVersions endpoints for each model type
- Centralize event handler binding in DownloadManager.js (no more inline HTML handlers)
- Modal UI and logic now auto-adapt to the current model type, making future extension easier

templates/checkpoints.html

@@ -12,7 +12,6 @@
 {% block init_check_url %}/api/checkpoints?page=1&page_size=1{% endblock %}
 
 {% block additional_components %}
-{% include 'components/checkpoint_modals.html' %}
 
 <div id="checkpointContextMenu" class="context-menu" style="display: none;">
     <!-- <div class="context-menu-item" data-action="details"><i class="fas fa-info-circle"></i> View Details</div> -->