Mount-gate SheetPortal for SSR safety

Defensive guard so the portal only mounts client-side. Radix's Portal is
already SSR-safe, so this is belt-and-suspenders.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RCfQKfr64YJbFRs3Ai9v2

components/ui/sheet.tsx

@@ -25,6 +25,15 @@ function SheetClose({
 function SheetPortal({
   ...props
 }: React.ComponentProps<typeof SheetPrimitive.Portal>) {
+  // Radix's Portal calls createPortal(children, document.body) as soon as it
+  // renders. During static prerendering there's no `document`, so gate the
+  // portal behind an effect that only runs in the browser. Closed sheets
+  // render nothing on the server anyway, so there's no visual/hydration change.
+  const [mounted, setMounted] = React.useState(false)
+  React.useEffect(() => setMounted(true), [])
+
+  if (!mounted) return null
+
   return <SheetPrimitive.Portal data-slot="sheet-portal" {...props} />
 }