feat: Add comprehensive backend validation, toast notifications, and Tokyo Night theme

- Backend: Add server-side validation with go-playground/validator/v10
- Frontend: Add toast notifications for API errors with @zerodevx/svelte-toast
- UI: Complete Tokyo Night theme redesign with modern animations
- Docs: Update COMPLETE_DOCUMENTATION.md and README.md with all enhancements
- Validation: Email format, password strength, and input sanitization
- UX: Real-time error feedback, loading states, and responsive design

bruno/auth/Register User.yml

@@ -0,0 +1,31 @@
+meta:
+  name: Register User
+  seq: 1
+http:
+  method: post
+  url: "{{base_url}}/api/auth/register"
+  body:
+    type: json
+    data: |
+      {
+        "email": "test@example.com",
+        "username": "testuser",
+        "password": "password123"
+      }
+docs: |
+  ## Register User
+
+  Creates a new user account with email, username, and password.
+
+  **Request Body:**
+  - `email` (string, required): Valid email address
+  - `username` (string, required): Unique username (3-50 chars)
+  - `password` (string, required): Password (min 6 chars)
+
+  **Response:**
+  - `token` (string): JWT authentication token
+  - `user` (object): User profile with id, email, username
+
+  **Error Responses:**
+  - 409: Email or username already exists
+  - 400: Invalid request data