fix: use config paths for static file mounts, redesign knowledge graph

- Fix generated images 404: use settings.generated_images_dir and
  settings.upload_dir instead of hardcoded relative paths in main.py
- Redesign knowledge graph with p5-style particle background, glow
  nodes, and curved gradient links
- Update course-content page border styling

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/main.py

@@ -139,12 +139,12 @@ app.include_router(course_content.router)
 app.include_router(forum.router)
 
 # 静态文件服务
-if os.path.exists("uploads"):
-    app.mount("/uploads", StaticFiles(directory="uploads"), name="uploads")
+if os.path.exists(settings.upload_dir):
+    app.mount("/uploads", StaticFiles(directory=settings.upload_dir), name="uploads")
 
 # 挂载生成图像目录
-if os.path.exists("generated_images"):
-    app.mount("/generated_images", StaticFiles(directory="generated_images"), name="generated_images")
+if os.path.exists(settings.generated_images_dir):
+    app.mount("/generated_images", StaticFiles(directory=settings.generated_images_dir), name="generated_images")
 
 # 根路径
 @app.get("/")