fix(ui): defer the visual-viewport read out of render

Reading window during render made the client's first render disagree with
the SSR markup; start at zero and fill in after mount, matching Mantine's
useViewportSize contract.

src/features/core/hooks/use-visual-viewport-size.ts

@@ -5,11 +5,13 @@ const eventListerOptions = {
 };
 
 const useVisualViewportSize = () => {
-  const windowExists = typeof window !== 'undefined';
+  // Starts at zero on server and client alike, filling in after mount — the
+  // same contract as Mantine's useViewportSize. Reading window during render
+  // makes the client's first render disagree with the SSR markup.
   const [windowSize, setWindowSize] = useState({
-    width: windowExists ? window.visualViewport?.width || 0 : 0,
+    width: 0,
-    height: windowExists ? window.visualViewport?.height || 0 : 0,
+    height: 0,
-    top: windowExists ? window.visualViewport?.offsetTop || 0 : 0,
+    top: 0,
   });
   const rafRef = useRef<number | null>(null);