Share session poll

“Share poll” button in the scheduling widget header. Copies "📅 Vote on our next [Campaign Name] session! → [URL]" to clipboard via the existing clipboard Stimulus controller. Button visible to all members when a poll is open (rendered above tabs). OG fallback: og:image now has a sitewide default (/android-chrome-512x512.png) so campaign share links always produce a preview card in Messenger/WhatsApp/iMessage.