fix: chat page not full width, scrollbar only works in middle area

[Alanaysis]

Description

Fix chat page layout issue: the chat content was constrained to max-width 960px centered, causing the scrollbar to appear in the middle of the page and scroll wheel only working in the middle area, with

meaningless blank space on both sides. This change moves the scroll container to full width while keeping message content and input box at the original 960px centered width, so scrollbar appears at the far
right and works across the entire page area without changing any visual appearance.

### Module(s) Affected
- [x] `web` (Frontend)

### Checklist
- [x] I have read and followed the contribution guidelines.
- [x] My code follows the project's coding standards.
- [ ] I have run `pre-commit run --all-files` and fixed any issues. (Pure CSS/HTML class change, no logic modified, no formatting issues)
- [ ] I have added relevant tests for my changes. (UI layout bugfix, no functional changes, no tests needed)
- [ ] I have updated the documentation (if necessary). (UI bugfix, no docs update needed)
- [x] My changes do not introduce any new security vulnerabilities.

### Additional Notes
The fix keeps all original UI appearance unchanged (message width, centered alignment, input box style all remain exactly the same), only moves the scrollbar to the far right edge of the page and allows

scrolling anywhere in the chat area.