feat(01-15): singleton PiP session with body-level custodial host

usePipSession() owns an off-screen div under document.body; adopt(video)
moves the element there so it survives the unmount of whatever view
rendered it (a Teleport and a KeepAlive'd view both move their subtree on
deactivation, which the picture-in-picture spec treats as removal).
release() tears down playback and detaches the element; a
leavepictureinpicture listener on the adopted element is the primary
release path so an orphaned owner can never leak it.

Also adds isPipSupported() to pip.ts — a call-time version of the existing
import-time pipSupported const, needed because a test can't restub
document.pictureInPictureEnabled after import. togglePip and pipSupported
are unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

neode-ui/src/utils/pip.ts

@@ -8,6 +8,17 @@ export const pipSupported =
   'pictureInPictureEnabled' in document &&
   document.pictureInPictureEnabled
 
+/** Same three checks as `pipSupported`, but evaluated at call time rather
+ *  than import time — lets a test stub support before or after this module
+ *  is imported, and lets a component re-check support on demand. */
+export function isPipSupported(): boolean {
+  return (
+    typeof document !== 'undefined' &&
+    'pictureInPictureEnabled' in document &&
+    document.pictureInPictureEnabled
+  )
+}
+
 export async function togglePip(video: HTMLVideoElement | null | undefined): Promise<void> {
   if (!video || !pipSupported) return
   try {