Made sidebar have fixed position

[KollPotato]

Fixes #338.

Works on my machine, here is proof:

fixed.sidebar.video.mp4

[Robyt3]

How does it work on mobile?

[KollPotato]

I have tested it in the dev tools on my desktop and also on my phone, works exactly like before.

[KollPotato]

YouCut_20250823_191335036.mp4

[KollPotato]

DON'T MERGE!!!! It does not behave as expected on some pages.

[KollPotato]

I changed so if there is an overflow the sidebar is scrollable, otherwise no scrollbar is shown.

[Robyt3]

The navigation scrollbar overlapping the content doesn't look good to me. The scrollbar being visible with a common screen size like 1920x1080 also doesn't look good.

[KollPotato]

The scrollbar being visible with a common screen size like 1920x1080 also doesn't look good.

It is intentional that scrollbar only shows up when content doesn't fit screen height

[Robyt3]

This now applies a transition: 0.4s; property to all elements, so the navigation position weirdly transitions when you resize the window, and highlighting of hovered links is barely visible anymore due to the slow transition.