fix: render node-update changelog notes as formatted markdown (#1474)

The Changelog tab in the Node Updates sheet rendered GitHub release notes
as raw markdown inside a preformatted block, so headings, bullet lists,
and issue/commit links showed as literal markup and were hard to read.

Render the notes with a small react-markdown wrapper styled to the design
tokens (raw HTML is intentionally not rendered, keeping it safe). Also:

- Replace the bare loading spinner with a content-shaped skeleton.
- Add a graceful empty state when no notes are available, with a link to
  the online changelog.
- Add a "View on Sencho" link next to "View on GitHub".
- Stop the release-notes fetch from re-firing on every failure by tracking
  whether a fetch has settled, so a null result lands on the empty state
  instead of looping; Recheck resets it to force a fresh fetch.

frontend/package.json

@@ -54,8 +54,10 @@
     "react-day-picker": "^10.0.1",
     "react-dom": "^19.2.7",
     "react-is": "^19.2.7",
+    "react-markdown": "^10.1.0",
     "react-use-measure": "^2.1.7",
     "recharts": "^3.9.0",
+    "remark-gfm": "^4.0.1",
     "tailwind-merge": "^3.6.0",
     "tailwindcss-animate": "^1.0.7",
     "yaml": "^2.9.0"