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/api/apiConfig.js

@@ -103,13 +103,12 @@ export const MODEL_SPECIFIC_ENDPOINTS = {
         moveBulk: `/api/${MODEL_TYPES.LORA}/move_models_bulk`,
         getTriggerWordsPost: `/api/${MODEL_TYPES.LORA}/get_trigger_words`,
         civitaiModelByVersion: `/api/${MODEL_TYPES.LORA}/civitai/model/version`,
-        civitaiModelByHash: `/api/${MODEL_TYPES.LORA}/civitai/model/hash`
+        civitaiModelByHash: `/api/${MODEL_TYPES.LORA}/civitai/model/hash`,
     },
     [MODEL_TYPES.CHECKPOINT]: {
-        info: `/api/${MODEL_TYPES.CHECKPOINT}/info`
+        info: `/api/${MODEL_TYPES.CHECKPOINT}/info`,
     },
     [MODEL_TYPES.EMBEDDING]: {
-        // Future embedding-specific endpoints
     }
 };