webui: fix a layout corner case where top navbar is hiding navtabs

[frb121]

At a certain display width the top navigation bar can overlap
navigation tabs below, this commit fixes the issue by adjusting
padding-top slightly.

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[pstorz]

The PR Title is too general, this can mean anything. It would make sense to describe what corner case is fixed.

E.g. Fix layout corner case wherer submenu is hidden under second line of main menu when main menu is wrapped.

[frb121]

The PR Title is too general, this can mean anything. It would make sense to describe what corner case is fixed.

E.g. Fix layout corner case wherer submenu is hidden under second line of main menu when main menu is wrapped.

I've adjusted the PR title to be more precise.

[pstorz]

Looks Good!