feat(sidebar): show empty folders and create folders from the sidebar (#999)

Empty folders (tracked in the scan-recorded all_folders list, same source
the move/download destination picker uses) can now be surfaced in the
folder sidebar via a view-options toggle, dimmed when their subtree holds
no models. Folders can be created directly from the sidebar through a new
POST /api/lm/{prefix}/create-folder endpoint with library-root
containment checks; the scanner records the new directory incrementally
so the tree reflects it without a rescan.

The sidebar header moves its view toggles (tree/list, recursive, empty
folders) into a "..." menu to fit the new create-folder button.

static/js/api/apiConfig.js

@@ -22,6 +22,7 @@ export const MODEL_CONFIG = {
         supportsLetterFilter: true,
         supportsBulkOperations: true,
         supportsMove: true,
+        supportsFolderManagement: true,
         templateName: 'loras.html'
     },
     [MODEL_TYPES.CHECKPOINT]: {
@@ -31,6 +32,7 @@ export const MODEL_CONFIG = {
         supportsLetterFilter: false,
         supportsBulkOperations: true,
         supportsMove: true,
+        supportsFolderManagement: true,
         templateName: 'checkpoints.html'
     },
     [MODEL_TYPES.EMBEDDING]: {
@@ -40,6 +42,7 @@ export const MODEL_CONFIG = {
         supportsLetterFilter: true,
         supportsBulkOperations: true,
         supportsMove: true,
+        supportsFolderManagement: true,
         templateName: 'embeddings.html'
     },
     [MODEL_TYPES.OTHER]: {
@@ -49,6 +52,7 @@ export const MODEL_CONFIG = {
         supportsLetterFilter: false,
         supportsBulkOperations: true,
         supportsMove: true,
+        supportsFolderManagement: true,
         templateName: 'other.html'
     }
 };
@@ -79,6 +83,7 @@ export function getApiEndpoints(modelType) {
         // Move operations (now common for all model types that support move)
         moveModel: `/api/lm/${modelType}/move_model`,
         moveBulk: `/api/lm/${modelType}/move_models_bulk`,
+        createFolder: `/api/lm/${modelType}/create-folder`,
 
         // CivitAI integration
         fetchCivitai: `/api/lm/${modelType}/fetch-civitai`,