fix(ui): seed useIsMobile from an SSR-safe snapshot to avoid the first-paint desktop flash

The previous useEffect-driven flavour returned false on the server and
on the first client paint, then flipped on the effect tick.
ResponsiveSheet and CoachDrawer branch their render tree off the value,
so a phone-class viewport painted the desktop branch for one frame
before swapping — Coach drawer side flipped from right to bottom
between paints and sheet contents re-mounted through the wrong layout.

Switch to useSyncExternalStore with a matchMedia client snapshot. The
server snapshot stays false (no window present) so SSR markup remains
deterministic, but the very first client render reads the live media
query state synchronously — by the time either consumer opens its
tree the hook already reflects the runtime viewport.

src/hooks/use-is-mobile.ts

@@ -1,37 +1,69 @@
 "use client";
 
-import { useEffect, useState } from "react";
+import { useSyncExternalStore } from "react";
 
 /**
- * SSR-safe viewport-width hook. Returns `true` once the browser
- * reports a viewport narrower than the requested Tailwind
- * breakpoint. The hook always returns `false` on the server and
- * on the first client paint so SSR markup matches the desktop
- * branch of any viewport-conditional render; the hook then flips
- * on the effect tick once `matchMedia` reports the live value.
+ * Viewport-width hook seeded from a SSR-safe snapshot.
  *
- * Defaults to the `md` breakpoint (768 px) because that matches
- * the existing dashboard `md:hidden` / `md:block` switches and
- * the `<ResponsiveSheet>` primitive's bottom-sheet branch.
- * Consumers that need a tighter cut (e.g. the Coach drawer, which
- * flips to a bottom-sheet only below `sm` / 640 px) pass an
- * explicit breakpoint.
+ * v1.4.27 R4 RC3 — the previous `useEffect`-driven flavour returned
+ * `false` on the server AND on the first client paint, then flipped to
+ * the live value on the effect tick. Two consumers
+ * (`<ResponsiveSheet>`, `<CoachDrawer>`) branch their render tree off
+ * the value, so a phone-class viewport painted the desktop branch for
+ * one frame before swapping. The Coach drawer's `side` flipped from
+ * `right` to `bottom` between paints; sheet contents re-mounted and
+ * controlled inputs flashed through the wrong layout.
+ *
+ * The new implementation uses `useSyncExternalStore` with a
+ * `matchMedia(...).matches` client snapshot. SSR still resolves to
+ * `false` (no `window`), but the very first client render reads the
+ * live media-query state synchronously, so the first paint already
+ * matches the runtime viewport.
+ *
+ * Trade-off: SSR markup for phone-class viewports still paints the
+ * desktop branch (no React hydration mismatch — both server and the
+ * client's first render under the SSR boundary use `false`), then the
+ * first client render after hydration flips to the live value. For the
+ * two consumers above this is acceptable because both gate visibility
+ * on `open`/`isOpen` state — the desktop branch never paints until the
+ * user actually opens the sheet/drawer, by which point hydration has
+ * already settled and the hook reads the live value on the very first
+ * render of the open tree.
+ *
+ * Defaults to the `md` breakpoint (768 px) because that matches the
+ * existing dashboard `md:hidden` / `md:block` switches and the
+ * `<ResponsiveSheet>` primitive's bottom-sheet branch. Consumers that
+ * need a tighter cut (e.g. the Coach drawer, which flips to a
+ * bottom-sheet only below `sm` / 640 px) pass an explicit breakpoint.
  */
-export function useIsMobile(breakpoint: "sm" | "md" = "md"): boolean {
-  const [isMobile, setIsMobile] = useState<boolean>(false);
+function getMediaQuery(breakpoint: "sm" | "md"): string {
+  const maxWidth = breakpoint === "sm" ? "639.98px" : "767.98px";
+  return `(max-width: ${maxWidth})`;
+}
+
+function subscribe(query: string): (callback: () => void) => () => void {
+  return (callback) => {
+    if (typeof window === "undefined") return () => {};
+    const mql = window.matchMedia(query);
+    mql.addEventListener("change", callback);
+    return () => mql.removeEventListener("change", callback);
+  };
+}
+
+function getClientSnapshot(query: string): boolean {
+  if (typeof window === "undefined") return false;
+  return window.matchMedia(query).matches;
+}
 
-  useEffect(() => {
-    if (typeof window === "undefined") return;
-    const maxWidth = breakpoint === "sm" ? "639.98px" : "767.98px";
-    const mql = window.matchMedia(`(max-width: ${maxWidth})`);
-    const sync = () => setIsMobile(mql.matches);
-    sync();
-    // `addEventListener` is the modern API; the older `addListener`
-    // is kept around for legacy Safari but every browser we ship to
-    // supports the typed event-listener form.
-    mql.addEventListener("change", sync);
-    return () => mql.removeEventListener("change", sync);
-  }, [breakpoint]);
+function getServerSnapshot(): boolean {
+  return false;
+}
 
-  return isMobile;
+export function useIsMobile(breakpoint: "sm" | "md" = "md"): boolean {
+  const query = getMediaQuery(breakpoint);
+  return useSyncExternalStore(
+    subscribe(query),
+    () => getClientSnapshot(query),
+    getServerSnapshot,
+  );
 }