feat: add groupIcon property to modelSpecs for custom group icons

Added the ability to define icons for custom model spec groups in the UI selector.

Changes:
- Added  property to TModelSpec type and schema in data-provider
- Created GroupIcon component to render URL or built-in endpoint icons
- Updated CustomGroup component to display group icons
- Added documentation and examples in librechat.example.yaml

Usage:

The groupIcon can be:
- A built-in endpoint key (e.g., "openAI", "anthropic", "groq")
- A URL to a custom icon image

Only the first spec in a group needs groupIcon - all specs share the same icon.

packages/data-provider/src/models.ts

@@ -22,6 +22,12 @@ export type TModelSpec = {
    * - If omitted, the spec appears as a standalone item at the top level
    */
   group?: string;
+  /**
+   * Optional icon URL for the group this spec belongs to.
+   * Only needs to be set on one spec per group - the first one found with a groupIcon will be used.
+   * Can be a URL or an endpoint name to use its icon.
+   */
+  groupIcon?: string | EModelEndpoint;
   showIconInMenu?: boolean;
   showIconInHeader?: boolean;
   iconURL?: string | EModelEndpoint; // Allow using project-included icons
@@ -40,6 +46,7 @@ export const tModelSpecSchema = z.object({
   default: z.boolean().optional(),
   description: z.string().optional(),
   group: z.string().optional(),
+  groupIcon: z.union([z.string(), eModelEndpointSchema]).optional(),
   showIconInMenu: z.boolean().optional(),
   showIconInHeader: z.boolean().optional(),
   iconURL: z.union([z.string(), eModelEndpointSchema]).optional(),