Recalculate sidebar position when annotator styles load

[robertknight]

If the annotator.css bundle loads after the sidebar is opened, the margin-left property on the .sidebar-container element can be set incorrectly. Fix this problem by invoking Sidebar._onResize when styles finish loading. This is the same way we handle "resize" events on the window.

Part of #7307.

Testing:

1. Apply this diff to simulate slow style bundle loading:

diff --git a/src/annotator/util/shadow-root.ts b/src/annotator/util/shadow-root.ts
index 18ef86a01..5a6218888 100644
--- a/src/annotator/util/shadow-root.ts
+++ b/src/annotator/util/shadow-root.ts
@@ -36,10 +36,13 @@ function loadStyles(shadowRoot: ShadowRoot): Promise<void> {
 
   const linkEl = document.createElement('link');
   linkEl.rel = 'stylesheet';
-  linkEl.href = url;
   linkEl.crossOrigin = 'anonymous';
   shadowRoot.appendChild(linkEl);
 
+  setTimeout(() => {
+    linkEl.href = url;
+  }, 1000);
+
   // When styles are loaded for the first time, wait for the stylesheet to load,
   // then extract `@property` declarations and append them to a stylesheet in
   // the top-level document.

The minimum timeout delay needed to trigger the problem varies. Locally I need it to be at least 100.

2. Visit http://localhost:3000.

On main, the sidebar ends up positioned on the left side. On this branch it will initially appear there, but then update its position once styles loading.

There is further improvement needed here, as ideally we want to prevent the sidebar from being displayed until styles have loaded. That will need more refactoring, and I think it is valuable to get this partial fix out first.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.47%. Comparing base (8a7d7a4) to head (b256726).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7309   +/-   ##
=======================================
  Coverage   99.47%   99.47%           
=======================================
  Files         271      271           
  Lines       10949    10954    +5     
  Branches     2616     2616           
=======================================
+ Hits        10891    10896    +5     
  Misses         58       58           

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