chore(dev): react-grab viewport sync helper for in-page debug toolbar

Mounts a dev-only client component that syncs the react-grab debug
toolbar's pinned edge / collapsed state across viewport changes (so
the toolbar doesn't drift off-screen when resizing or rotating).

Render is gated by NODE_ENV === 'development' in src/app/layout.tsx;
production builds tree-shake the import out via process.env replacement.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/layout.tsx

@@ -4,6 +4,7 @@ import { headers } from 'next/headers';
 import { Inter, JetBrains_Mono } from 'next/font/google';
 import { Toaster } from 'sonner';
 import { classifyFormFactor } from '@/lib/form-factor';
+import { ReactGrabViewportSync } from '@/components/dev/react-grab-viewport-sync';
 import './globals.css';
 
 const inter = Inter({
@@ -66,6 +67,7 @@ export default async function RootLayout({ children }: { children: React.ReactNo
       >
         {children}
         <Toaster richColors position="top-right" />
+        {process.env.NODE_ENV === 'development' && <ReactGrabViewportSync />}
       </body>
     </html>
   );

src/components/dev/react-grab-viewport-sync.tsx

@@ -0,0 +1,104 @@
+'use client';
+
+import { useEffect } from 'react';
+
+type Edge = 'top' | 'bottom' | 'left' | 'right';
+
+interface ToolbarState {
+  edge: Edge;
+  ratio: number;
+  collapsed: boolean;
+  enabled: boolean;
+  defaultAction?: string;
+}
+
+interface ReactGrabAPI {
+  setToolbarState: (state: Partial<ToolbarState>) => void;
+  onToolbarStateChange: (cb: (state: ToolbarState) => void) => () => void;
+}
+
+declare global {
+  interface Window {
+    __REACT_GRAB__?: ReactGrabAPI;
+  }
+}
+
+const MOBILE_QUERY = '(max-width: 1023.98px)';
+const DESKTOP_KEY = 'react-grab-toolbar-state-desktop';
+const MOBILE_KEY = 'react-grab-toolbar-state-mobile';
+
+const DESKTOP_DEFAULT: Partial<ToolbarState> = {
+  edge: 'bottom',
+  ratio: 0.5,
+  collapsed: false,
+};
+
+const MOBILE_DEFAULT: Partial<ToolbarState> = {
+  edge: 'right',
+  ratio: 0.5,
+  collapsed: false,
+};
+
+export function ReactGrabViewportSync() {
+  useEffect(() => {
+    if (process.env.NODE_ENV !== 'development') return;
+
+    const cleanups: Array<() => void> = [];
+    let pollId: number | undefined;
+
+    const wireUp = (api: ReactGrabAPI) => {
+      const mql = window.matchMedia(MOBILE_QUERY);
+      const keyFor = () => (mql.matches ? MOBILE_KEY : DESKTOP_KEY);
+      const defaultFor = () => (mql.matches ? MOBILE_DEFAULT : DESKTOP_DEFAULT);
+
+      let suppressNextWrite = false;
+      const apply = () => {
+        const stored = localStorage.getItem(keyFor());
+        suppressNextWrite = true;
+        api.setToolbarState(stored ? (JSON.parse(stored) as ToolbarState) : defaultFor());
+      };
+
+      apply();
+
+      const unsubscribe = api.onToolbarStateChange((state) => {
+        if (suppressNextWrite) {
+          suppressNextWrite = false;
+          return;
+        }
+        localStorage.setItem(keyFor(), JSON.stringify(state));
+      });
+
+      mql.addEventListener('change', apply);
+      cleanups.push(unsubscribe, () => mql.removeEventListener('change', apply));
+    };
+
+    const tryWire = () => {
+      const api = window.__REACT_GRAB__;
+      if (!api) return false;
+      wireUp(api);
+      return true;
+    };
+
+    if (!tryWire()) {
+      pollId = window.setInterval(() => {
+        if (tryWire() && pollId !== undefined) {
+          window.clearInterval(pollId);
+          pollId = undefined;
+        }
+      }, 100);
+      window.setTimeout(() => {
+        if (pollId !== undefined) {
+          window.clearInterval(pollId);
+          pollId = undefined;
+        }
+      }, 5000);
+    }
+
+    return () => {
+      if (pollId !== undefined) window.clearInterval(pollId);
+      cleanups.forEach((fn) => fn());
+    };
+  }, []);
+
+  return null;
+}