feat(admin): per-feed Subscribe chips with copy/open/validate for RSS/Atom/JSON

Replace the stacked RSS/Atom URL rows in the dashboard with a compact
"Subscribe" chip block exposing all three feed formats — including JSON
Feed, previously absent from the admin UI. Each chip carries copy, open,
and validate actions; validation links to the W3C Feed Validator (RSS/Atom)
and validator.jsonfeed.org (JSON). The Table view's RSS+Atom columns fold
into a single Formats column.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

README.md

@@ -16,6 +16,7 @@ kill-the-news keeps the same workflow while avoiding shared domains and shared d
 - Bulk feed/email deletion from the admin dashboard (safe checkbox-based flow)
 - Inline double-confirm delete interactions with toast feedback in the admin dashboard
 - Resizable + sortable table columns in the admin dashboard (Table view)
+- Per-feed "Subscribe" chips in the admin dashboard — copy, open, or validate the feed in one click for each of RSS, Atom, and JSON Feed (validation via the W3C Feed Validator and validator.jsonfeed.org)
 - Unique newsletter addresses per feed (for example `[email protected]`)
 - **Separate inbound address and feed URL** — the address you subscribe with (`[email protected]`) and the public feed URL (`/rss/<opaque-id>`) use **independent** ids, so you can share a feed without leaking the address that feeds it, and an address harvested by a newsletter can't be used to read your feed (`/rss/<your-address>` 404s)
 - Cloudflare Email Workers ingestion (no third-party service)