fix: navbar's messy layout at narrow screen

[ulivz]

Summary

Solve the navbar's messy layout under the narrow screen, also a replacement of #266

Here's an optimization point that we can continue to do. When navbar's links are too long, some nav links on the right side may not be visible. But at the moment I have not come up with a good solution, or the solution I came up with is more complicated. The current proposal is to control the width of the sidebar.

Before

After

cc @chrisvfritz

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