Add ability to fetch and inject web page content into chat as context.
Includes SSRF protection (blocks private IPs, localhost), content
extraction (strips scripts/styles/nav), and a clean popover UI.
Reimplements the concept from PR #1703 without the issues (duplicated
ChatBox, dual API routes, SSRF vulnerability, window.prompt UX).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>