site: fix dev-server 404 on scoped feed URLs

With trailingSlash 'always', astro dev only matches dynamic-route
endpoints when the request has a trailing slash, so the RSS pill links
(/tags/<t>/rss.xml, /categories/<c>/rss.xml) 404'd locally while the
built site served them fine. Dev-only vite middleware rewrites those
requests to the slashed form.

Claude-Session: https://claude.ai/code/session_01WZaczDJjL3xZ3u5spsN5AL

astro.config.mjs

@@ -128,11 +128,26 @@ const setPickApi = {
   },
 };
 
+// With trailingSlash 'always', the dev server only matches dynamic-route
+// endpoints (/tags/<t>/rss.xml, /categories/<c>/rss.xml) when the request
+// has a trailing slash; the built site serves them as plain files without
+// one. Rewrite in dev so the same URLs work in both.
+const feedSlashShim = {
+  name: 'feed-trailing-slash-dev-shim',
+  apply: 'serve',
+  configureServer(server) {
+    server.middlewares.use((req, res, next) => {
+      if (/^\/(tags|categories)\/[^/]+\/rss\.xml$/.test(req.url)) req.url += '/';
+      next();
+    });
+  },
+};
+
 export default defineConfig({
   site: 'https://unique.rzen.dev',
   trailingSlash: 'always',
   build: { format: 'directory' },
-  vite: { plugins: [editFindTagApi, setPickApi] },
+  vite: { plugins: [editFindTagApi, setPickApi, feedSlashShim] },
   integrations: [
     mdx(),
     sitemap({