WIP: Adjustable border for Library side panel

[ormsbee]

Lets users move the side panel border back and forth. It's especially helpful with Unit and Component content previews.

BEFORE

AFTER

Also has implications for when you're adding library content from a course.

BEFORE

AFTER

This is in draft because it's all written by Claude, and it's going to take me a while to go through it to make sure I actually understand what's going on (because I'm very bad at frontend).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.59%. Comparing base (fc51f17) to head (e0ccd0e).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3070      +/-   ##
==========================================
+ Coverage   95.57%   95.59%   +0.02%     
==========================================
  Files        1393     1392       -1     
  Lines       33029    33060      +31     
  Branches     7662     7659       -3     
==========================================
+ Hits        31568    31605      +37     
+ Misses       1408     1389      -19     
- Partials       53       66      +13     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.