fix(stories): portal the story overlay to escape overflow-auto ancestor

Real root cause of the mobile rendering bug (confirmed via an actual
Android Chrome screenshot, not devtools emulation): StoryViewer's
fixed inset-0 div is a descendant of <main className="overflow-auto">
in the (app) layout. On Android Chrome, a position:fixed element
inside an overflow-auto ancestor can size itself against that
ancestor's full scrollable content instead of the real viewport —
the overlay rendered many times taller than the screen, so only a
small top slice was visible, looking like a heavily zoomed crop. The
previous two commits were chasing the video's object-fit/aspect-ratio,
which was never the actual problem.

Renders the overlay via createPortal to document.body instead, the
same pattern Radix/Base UI's Sheet and Dialog already use internally
for exactly this class of bug.

src/components/stories/StoryViewer.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { useState, useEffect, useCallback, useRef } from "react";
+import { createPortal } from "react-dom";
 import dynamic from "next/dynamic";
 import { X, Eye, Volume2, VolumeX, Loader2 } from "lucide-react";
 import { useTranslations } from "next-intl";
@@ -65,6 +66,12 @@ export function StoryViewer({
   const [isMuted, setIsMuted] = useState(true);
   const advancedRef = useRef(false);
 
+  // Portal target must wait for mount (document isn't available during SSR).
+  const [mounted, setMounted] = useState(false);
+  useEffect(() => {
+    setMounted(true);
+  }, []);
+
   const { data: stories = [] } = useQuery(
     trpc.stories.listActive.queryOptions({ petId })
   );
@@ -183,8 +190,14 @@ export function StoryViewer({
 
   return (
     <>
-      {/* Full-screen overlay */}
-      {open && (
+      {/* Full-screen overlay — portaled to document.body so `position: fixed`
+          escapes the (app) layout's <main className="overflow-auto"> ancestor.
+          A fixed descendant of an overflow-auto container can, on some mobile
+          browsers (confirmed on Android Chrome), size itself against that
+          container's full scrollable content instead of the real viewport —
+          the story then renders many times taller than the screen and only
+          a small top slice is visible, looking like a heavily zoomed crop. */}
+      {open && mounted && createPortal(
         <div
           className="fixed inset-0 z-50 bg-black/90 flex items-center justify-center"
           role="dialog"
@@ -337,7 +350,8 @@ export function StoryViewer({
               <span>{viewers.length}</span>
             </button>
           )}
-        </div>
+        </div>,
+        document.body
       )}
 
       {/* Viewer list sheet — owner only */}