Add page client state sharing for feed components

Feed components now share the selected feed ID via page client state
(usePageClientValue), enabling automatic synchronization when using
PvFeedSelector with PvFeedNewCount, PvFeedEventList, and PvFeedManagement
on the same MDX page without explicit prop passing.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>