Adds a URL input + Subscribe button (dx-components Input/Button) to the
UI, wired to a new subscribe_feed server function that upserts the feed
row and does an immediate first fetch so the reader isn't empty until
the next scheduled poll. feedsignal_feeds::fetch_feed now also returns
the feed's title, used to replace the URL placeholder once fetched.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XJJQb2DWZZwQ1yPiaAQQoY
