refactor: remove legacy frontend code and implement new Next.js structure

- Deleted the old Register page and utility functions.
- Removed Tailwind CSS configuration and Vite configuration files.
- Added a new script for starting a single container with FastAPI and Next.js.
- Updated README to reflect the current status of the Next.js frontend.
- Implemented new login and registration API routes with improved error handling.
- Refactored frontend API calls to use the new proxy structure.
- Enhanced error handling in API response processing.
- Updated components to align with the new API endpoints and structure.

docker-compose-single.yml

@@ -1,8 +1,6 @@
 # ==================== 单容器部署配置 ====================
 # 使用方法：docker-compose -f docker-compose-single.yml up -d
 
-version: '3.8'
-
 services:
   qquiz:
     build:
@@ -34,4 +32,8 @@ services:
 
 volumes:
   qquiz_data:
+    # Reuse the previous split-stack SQLite volume during migration.
+    name: qquiz_sqlite_data
   qquiz_uploads:
+    # Reuse the previous split-stack uploads volume during migration.
+    name: qquiz_upload_files