feat: improve mobile navbar

[billyyyyy3320]

Summary

I found the font-family of navlink should be PT Serif, Serif, but it use VP's default font-family in mobile screen. Besides, navlinks in mobile are too narrow.

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Docs
• [ ] Build-related changes
• [ ] Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No