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

static/js/checkpoints.js

@@ -2,7 +2,6 @@ import { appCore } from './core.js';
 import { confirmDelete, closeDeleteModal, confirmExclude, closeExcludeModal } from './utils/modalUtils.js';
 import { createPageControls } from './components/controls/index.js';
 import { loadMoreCheckpoints } from './api/checkpointApi.js';
-import { CheckpointDownloadManager } from './managers/CheckpointDownloadManager.js';
 import { CheckpointContextMenu } from './components/ContextMenu/index.js';
 import { ModelDuplicatesManager } from './components/ModelDuplicatesManager.js';
 import { MODEL_TYPES } from './api/apiConfig.js';
@@ -13,9 +12,6 @@ class CheckpointsPageManager {
         // Initialize page controls
         this.pageControls = createPageControls(MODEL_TYPES.CHECKPOINT);
         
-        // Initialize checkpoint download manager
-        window.checkpointDownloadManager = new CheckpointDownloadManager();
-        
         // Initialize the ModelDuplicatesManager
         this.duplicatesManager = new ModelDuplicatesManager(this, MODEL_TYPES.CHECKPOINT);