style: adjust media query breakpoint for student training styles

- Changed the media query breakpoint from 1100px to 800px to improve responsiveness of the student training shell on narrower screens.
- Ensured that the AI floating panel remains fixed and adapts its width appropriately on smaller devices.

src/assets/styles/student-training.scss

@@ -1064,7 +1064,7 @@ body.student-light-mode {
   white-space: nowrap !important;
 }
 
-@media (max-width: 1100px) {
+@media (max-width: 800px) {
   /* AI 悬浮球/面板在窄屏仍 fixed 悬浮；面板宽度自适应 100vw-32px */
 
   .student-training-shell .workbench-actions {