fix(react): use useLayoutEffect for hasMounted to prevent source element race condition

[pyto86pri]

Summary

• Use useLayoutEffect instead of useEffect for setHasMounted(true) in MediaOutlet to prevent a race condition with hls.js's removeSourceChildren()

Problem

On iOS 17+, hls.js uses ManagedMediaSource which calls removeSourceChildren() during initialization, removing all <source> elements from the <video> element — including ones rendered by React (when !hasMounted).

The current useEffect for setHasMounted(true) triggers a batched re-render that may be deferred by React's scheduler (especially during unmount→remount transitions like Next.js page navigation). This allows the RAF-scheduled hls.js initialization to fire first, removing React's <source> elements before React can reconcile them, resulting in a NotFoundError from removeChild.

Timeline (before fix)

[commit]  <video><source>...</source></video>  (React tracks <source> in fiber tree)
[paint]   browser paint
[effect]  setHasMounted(true) → batched re-render scheduled
          React scheduler yields (>5ms due to unmount cleanup work)
[RAF]     hls.js init → removeSourceChildren() → removes React's <source>
[task]    React re-render → removeChild(<source>) → NotFoundError!

Timeline (after fix)

[commit]  <video><source>...</source></video>
[layout]  setHasMounted(true) → synchronous re-render → <source> removed
[paint]   browser paint
[RAF]     hls.js init → removeSourceChildren() → nothing to remove

Notes

• SSR behavior is preserved: useLayoutEffect does not run on the server, so <source> elements are still rendered in SSR HTML for preloading
• The useLayoutEffect re-render is lightweight (only removes <source> children), so no visible paint delay

Closes #1521