Adjust sidenotes to not rely on border width subtraction (#58)

Closes #26.

[As I outlined in the
issue:](#26 (comment))

> The reason you're still seeing the issue is that borders (and borders
in particular) are rounded by the browser to device pixel boundaries.
Thus, the computation for the [smaller padding to compensate for the
bigger border] doesn't exactly match the change in border width. The end
result is a barely-noticeable layout shift.

> There's no way to soundly fix this since rounding behavior depends on
display, pixel density, and zoom. Instead, I can change our style to
change the border color and add a drop shadow to highlight the
hovered-over note. This precludes the need for the computation I
described (because the border width now remains constant), and thus
removes any place where rounding can make its way in.

This PR does this.

| No Hover | Old Hover | New Hover |
|-|-|-|

|![notenohover](https://github.com/user-attachments/assets/df2e6830-2bd2-4f39-b750-1d6fb339a13b)|![noteoldhover](https://github.com/user-attachments/assets/d65ff3cd-a15d-4fde-9065-615ae2ded0ca)|![notenewhover](https://github.com/user-attachments/assets/9cef7c6f-ba53-42b8-8f1f-bd76b1909aed)|

I think it looks pretty nice:

<img width="1034" height="476" alt="Screenshot 2026-02-23 at 10 34
02 AM"
src="https://github.com/user-attachments/assets/bf17a322-2dfe-40d2-9cae-417cba8d9ac3"
/>

Reviewed by @benharsh -- thanks!

## Testing
- [x] hover no longer changes line breaks in sidenotes on any zoom level
I tried in the 10 myths, part 7 post from the original issue.

Author: DanilaFe

themes/chapel-theme/assets/scss/sidenotes.scss

@@ -3,7 +3,7 @@
 @import "margin.scss";
 
 $sidenote-padding: 1rem;
-$sidenote-highlight-border-width: .2rem;
+$sidenote-highlight-border-width: $standard-border-width;
 
 .sidenote {
     &:hover {
@@ -13,10 +13,11 @@ $sidenote-highlight-border-width: .2rem;
         }
 
         .sidenote-content {
-            border: $sidenote-highlight-border-width dashed;
+            border: $sidenote-highlight-border-width solid;
             padding: $sidenote-padding -
                 ($sidenote-highlight-border-width - $standard-border-width);
             border-color: $primary-color;
+            box-shadow: 0 0 0.4rem rgba(darken($primary-color, 20%), 0.3);
         }
     }
 }