refactor: Reorganize Bruno requests by role permissions

- Create separate admin folder for admin-only operations
- Move admin endpoints (Create/Update/Delete Ebook, Scanner, Folder Management) to /bruno/admin/
- Add admin dashboard, profile, and library page requests
- Add Register Admin User request with explicit admin role
- Update Register User request to include role field
- Remove empty scanner folder structure

API organization:
- Admin requests: /bruno/admin/ (require admin role)
- User requests: /bruno/user/ (available to all authenticated users)
- Environment: Single {{token}} variable works for both roles

bruno/ebooks/Delete Ebook.bru

@@ -1,40 +0,0 @@
-meta {
-  name: Delete Ebook
-  type: http
-  seq: 5
-}
-
-delete {
-  url: http://localhost:8765/api/ebooks/{{ebookid}}
-  body: none
-  auth: inherit
-}
-
-script:post-response {
-  function onResponse(res) {
-    let token = bru.setEnvVar("ebookid", "", { persist: true });
-  }
-  onResponse(res);
-}
-
-settings {
-  encodeUrl: true
-  timeout: 0
-}
-
-docs {
-  ## Delete Ebook
-  
-  Removes an ebook from the library.
-  
-  **Authentication:** Required (Bearer token)
-  
-  **Path Parameters:**
-  - `id` (string): Ebook UUID
-  
-  **Response:** 204 No Content
-  
-  **Error Responses:**
-  - 401: Invalid authentication
-  - 404: Ebook not found
-}