One border weight, and no sliver beside the nav

The nav's right edge read as two lines because it was two things: its own
border, and a sliver of the stage's background showing through a
sub-pixel gap before the app's webview began. The stage rect is now
rounded to whole pixels and to the same edges each time, so the app
covers the stage exactly.

Borders are one weight throughout and the lightest that still separates -
slate-200 in light, slate-800 in dark. Nothing drawn heavier than it
needs to be to read as an edge.

src/components/Nav.tsx

@@ -38,7 +38,7 @@ export default function Nav({
   const disabled = !activeId;
 
   const shell =
-    "flex shrink-0 flex-col overflow-hidden border-r border-slate-300 bg-white " +
+    "flex shrink-0 flex-col overflow-hidden border-r border-slate-200 bg-white " +
     "dark:border-slate-800 dark:bg-slate-900";
 
   /* Back, forward, reload and settings. Nothing hides an element here — that