site: minimal redesign with markdown rendering (#149)

Complete redesign of the changelog site: single-column, generous
whitespace, hairline dividers, no card chrome. The masthead eyebrow,
intro grid, side-panel update picker, and source footer are gone.

Update bodies are now markdown, rendered with `marked`, so inline code,
links, and emphasis render properly instead of showing literal
backticks. The data shape collapses to `body: string` plus `links[]`;
the RSS feed flattens markdown to a plain description through a small
`plainText` helper.

Author: andrewgazelka

site/package-lock.json

@@ -27,5 +27,8 @@
   },
   "overrides": {
     "cookie": "^0.7.0"
+  },
+  "dependencies": {
+    "marked": "^16.4.2"
   }
 }