fix(preview): clip drawer behind .content with position:relative+overflow:hidden

The drawer was bleeding its close-X out of the window's left side
because .content (its containing block) had display:grid but no
position:relative, so absolute-positioned children anchored to .window
or further up the tree. Adding position:relative makes the drawer
anchor to .content, and overflow:hidden clips the off-screen
translateX(100%) so its content doesn't render outside the bounds.

docs/preview/redesigned-mainwindow.html

@@ -174,6 +174,8 @@
     display: grid;
     grid-template-rows: 44px auto 1fr auto 32px;
     min-width: 0;
+    position: relative;
+    overflow: hidden;
   }
   .titlebar {
     display: grid;