feat: chat sharing UI and playbook viewer (/playbook/:uuid)

[nv78]

Summary

• Share button in the Chatbot input bar (shown to authenticated users with an active chat that has messages): clicking calls GET /generate-playbook/:chatId and displays a modal with a copy-to-clipboard shareable URL
• PlaybookView (/playbook/:uuid): public read-only page that renders a shared chat snapshot; includes an "Import to my chats" button that calls POST /playbook/:uuid and navigates the user to their new copy
• Backend: extended /retrieve-shared-messages-from-chat to accept playbook_url (UUID), using the existing retrieve_messages_from_share_uuid() DB function that was already implemented but had no HTTP endpoint

What already existed (backend, untouched)

• GET /generate-playbook/<chat_id> → creates a share record, returns /playbook/{uuid}
• POST /playbook/<uuid> → imports the shared chat into the requesting user's account
• create_chat_shareable_url(), access_shareable_chat(), retrieve_messages_from_share_uuid() in database/db.py

Test plan

• Open a chat with messages → share button appears in the input bar
• Click share → modal shows with a full URL (/playbook/some-uuid)
• Click Copy → URL is copied to clipboard, button shows checkmark
• Open the URL in an incognito window → PlaybookView loads the read-only chat
• Click "Log in to import" → redirects to root login
• Log in and open the playbook URL → "Import to my chats" appears
• Click Import → navigates to /chat/<new_chat_id> with all messages loaded

https://claude.ai/code/session_01C9mHttiQ4ZAaBbQecVV7uu