Announcements chat extension: card UI + opt-in broadcasts

[SBalaguer]

This PR rebuilds the chat extension from the plain-text/poll-loop bot into a card-rendered system with opt-in announcement broadcasts.

What changed

• Custom-rendered cards instead of plain text. Welcome/menu, latest announcements, talks, community sessions, time queries, plus error/loading states.
• Opt-in broadcasts ("Keep me posted"). When on, new on-chain announcements are posted into the chat: live via a ContractEmitted watcher on Android/Desktop, and via catch-up-on-open on iOS (the host suspends the connection in-chat there). Off by default. The feed stays pull-only.
• Text routing. No host emits Command actions or intercepts /, so commands are parsed from plain text.
• Snapshot-first reads. A stale-while-revalidate snapshot (memory → hostLocalStorage → live build) so cards render instantly even when the host has paused connections.
• Deeplinks posted as chat messages (map / full schedule) rather, given chat extensions still don't support navigateTo.
• The poll loop and /help are removed.

[github-actions]

Preview Deployment Ready!

App	Domain
Admin	pr25-manager
Festival	pr25-attendee

Deployment Details

App	CID
Admin	bafybeif73ohhogxk6kukqfdpoenhub7xvj64ptctmb4khtjq3sijopp5zu
Festival	bafybeie3icgtayuuztpgtsweu6kaaoiy5zxqazrsxiqwli5hvgvmh2svea

Info	Value
Commit	e4f526c1cf0480848b5dd5a61a0735a5ca46a4d7
Branch	sb/feat/chat-extension-redesign

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