Upgrade Astro 4 → 5; fix sitemap astro:routes:resolved crash

- Upgrade astro 4.16.19 → 5.17.3 and all official integrations
- Remove deprecated @astrojs/tailwind; replace with postcss.config.cjs
  so Tailwind v3 continues to work via Vite's built-in PostCSS support
- Rename ViewTransitions → ClientRouter in base-head.astro (Astro 5)
- Add autoprefixer as a devDependency for PostCSS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ctrimm

astro.config.mjs

@@ -1,7 +1,6 @@
 import { defineConfig } from "astro/config";
 import react from "@astrojs/react";
 import mdx from "@astrojs/mdx";
-import tailwind from "@astrojs/tailwind";
 import sitemap from "@astrojs/sitemap";
 import icon from "astro-icon";
 
@@ -40,9 +39,6 @@ export default defineConfig({
     }),
     sitemap(),
     react(),
-    tailwind({
-      applyBaseStyles: false,
-    }),
     icon(),
   ],
 });

package.json

@@ -24,13 +24,12 @@
     "sst:remove": "sst remove"
   },
   "dependencies": {
-    "@astrojs/mdx": "^2.1.1",
-    "@astrojs/node": "^8.3.4",
-    "@astrojs/react": "^3.0.9",
-    "@astrojs/rss": "^4.0.4",
-    "@astrojs/sitemap": "^3.1.5",
-    "@astrojs/tailwind": "^5.1.0",
-    "@astrojs/vercel": "^6.1.0",
+    "@astrojs/mdx": "^4.3.13",
+    "@astrojs/node": "^9.5.4",
+    "@astrojs/react": "^4.4.2",
+    "@astrojs/rss": "^4.0.15",
+    "@astrojs/sitemap": "^3.7.0",
+    "@astrojs/vercel": "^9.0.4",
     "@aws-sdk/client-dynamodb": "^3.699.0",
     "@iconify-json/lucide": "^1.2.92",
     "@iconify-json/mdi": "^1.2.3",
@@ -49,7 +48,7 @@
     "@types/aws-lambda": "^8.10.145",
     "@types/react": "^18.2.51",
     "@types/react-dom": "^18.2.18",
-    "astro": "^4.3.0",
+    "astro": "^5.17.3",
     "astro-icon": "^1.1.5",
     "balance-text": "^3.3.1",
     "class-variance-authority": "^0.7.0",
@@ -72,6 +71,7 @@
     "@tailwindcss/typography": "^0.5.10",
     "@tsconfig/node20": "^20.1.4",
     "@types/node": "^22.9.2",
+    "autoprefixer": "^10.4.24",
     "mdast-util-toc": "^7.0.0",
     "remark": "^15.0.1",
     "tsx": "^4.19.2",