gh-13857: Use Zen icon for share toolbar button (gh-13896)

Fixes #13857.

This updates the toolbar share button to use Zen's shared icon set
instead of the upstream Firefox asset. The downloads button already
points at the Zen icon set, so this keeps the two toolbar icons visually
consistent.

Validation:
- Checked that `share-tab-button` is the Firefox toolbar share button
id.
- Confirmed the repo lint script requires the generated `engine`
checkout, which is not present in this clone.

Co-authored-by: arieleli01212 <207917221+arieleli01212@users.noreply.github.com>

src/browser/themes/shared/zen-icons/icons.css

@@ -435,6 +435,7 @@
 }
 
 #zen-copy-current-url-button,
+#share-tab-button,
 #zen-site-data-header-share {
   list-style-image: url("share.svg");
 }