refactor: optimize RSS feed generation

Switches from getRssString to modern rss helper function
Moves XSL stylesheet to dedicated static file
Improves code organization and maintainability

Simplifies RSS feed configuration while maintaining beautification option

src/pages/rss.xml.js

@@ -1,8 +1,7 @@
-import { getRssString } from '@astrojs/rss'
+import rss from '@astrojs/rss'
 import sanitizeHtml from 'sanitize-html'
 import { getChannelInfo } from '../lib/telegram'
 import { getEnv } from '../lib/env'
-import style from '../assets/rss.xsl?raw'
 
 export async function GET(Astro) {
   const { SITE_URL } = Astro.locals
@@ -17,11 +16,12 @@ export async function GET(Astro) {
   url.pathname = SITE_URL
   url.search = ''
 
-  let rssString = await getRssString({
+  const response = await rss({
     title: `${tag ? `${tag} | ` : ''}${channel.title}`,
     description: channel.description,
     site: url.origin,
     trailingSlash: false,
+    stylesheet: getEnv(import.meta.env, Astro, 'RSS_BEAUTIFY') ? '/rss.xsl' : undefined,
     items: posts.map(item => ({
       link: `posts/${item.id}`,
       title: item.title,
@@ -42,15 +42,8 @@ export async function GET(Astro) {
     })),
   })
 
-  const enableBeautify = getEnv(import.meta.env, Astro, 'RSS_BEAUTIFY')
-  if (enableBeautify) {
-    rssString = rssString.replace(/^(<\?xml .*\?>)/i, style)
-  }
+  response.headers.set('Content-Type', 'text/xml')
+  response.headers.set('Cache-Control', 'public, max-age=3600')
 
-  return new Response(rssString, {
-    headers: {
-      'Content-Type': 'text/xml',
-      'Cache-Control': 'public, max-age=3600',
-    },
-  })
+  return response
 }