Let window chrome shrink

Now it's no worse than it was pre-redesign. But there's still
room for improvement, either by preventing the window from shrinking to the
point that the sidebars overflow or by making the sidebars horizontally
scrollable at very small window sizes.

scss/components/_mainWindow.scss

@@ -140,3 +140,8 @@
 :is(panel)::part(content) {
 	--panel-background: var(--material-toolbar);
 }
+
+#zotero-pane-stack > hbox {
+	// Let window chrome, tabs, and toolbar shrink even when sidebars overflow
+	min-width: 0;
+}