Improvement/sticky sidebar

[barna-isaac]

This implements sticky sidebars.

Here are some controversial choices I made:

1. I prioritised the case when the sidebar is short enough there’s no need for a scrollbar. In such a case, the page looks exactly as it did before. When there is a scrollbar, the sidebar’s contents is now narrower, to acommodate the width of the scrollbar. Maybe the reviewer knows a way to have the scrollbar eat into the padding we already have on the page? Unfortunately I couldn’t do this easily (considered relying on scrollbar-gutter: stable, but whether this takes up any space still depends on whether the os shows scrollbars).
2. when the main content becomes shorter, we loose scroll position and there’s a big jump. This might happen, eg. on the Question finder, see attached video. I don't think there's an easy fix for this one: the browser remembers we're on the bottom of the page, but with the page a lot shorter, the bottom of the page becomes the footer, which doesn't have the scrollbar.
   https://github.com/user-attachments/assets/480366b2-1030-4f96-80a8-bbde4d359f3c
3. For simplicity, I made every sidebar sticky. There are a few pages, like  "News", where sticky sidebars feel unnecessary, as the sidebar doesn't really contain anything meaningful in the first place.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.20%. Comparing base (e7e8ce7) to head (eab7600).
⚠️ Report is 408 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/app/components/elements/layout/SidebarLayout.tsx	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2293      +/-   ##
==========================================
- Coverage   43.52%   43.20%   -0.32%     
==========================================
  Files         597      601       +4     
  Lines       25251    25732     +481     
  Branches     7500     8567    +1067     
==========================================
+ Hits        10990    11118     +128     
- Misses      14212    14557     +345     
- Partials       49       57       +8     

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[barna-isaac]

More weirdness:

• on question finder search box, focus outline is now clipped
• pages with unexpected jumps: any page that might become shorter in response to a sidebar action. Question Finders, Concept Finders, Test finder, "My tests" (and probably a lot more!)