refactor: rename bruno/auth folder to bruno/user

The auth folder now contains user management endpoints beyond just authentication:
- User registration/login (auth)
- Profile management (user)
- Theme settings (user preferences)
- Ebook folder management (user settings)

Renaming to 'user' better reflects the expanded scope covering user accounts, preferences, and settings management.

bruno/user/Update Theme.bru

@@ -0,0 +1,40 @@
+meta {
+  name: Update Theme
+  type: http
+  seq: 4
+}
+
+put {
+  url: {{base_url}}/api/auth/theme
+  body: json
+  auth: inherit
+}
+
+body:json {
+  {
+    "theme": "dracula"
+  }
+}
+
+docs {
+  ## Update User Theme
+
+  Updates the authenticated user's theme preference.
+
+  **Method:** PUT
+
+  **Endpoint:** /api/auth/theme
+
+  **Authentication:** Required (Bearer token)
+
+  **Request Body:**
+  - `theme` (string): Theme name (tokyo-night, dracula, nord, solarized-dark, monokai, one-dark-pro, material-dark, catppuccin-mocha, catppuccin-macchiato, catppuccin-frappe, catppuccin-latte)
+
+  **Response:**
+  - `message` (string): Success message
+
+  **Status Codes:**
+  - 200: Success
+  - 400: Invalid theme
+  - 401: Unauthorized
+}