build(vercel): no-cache headers for HTML + service worker

Adds Cache-Control: no-cache,must-revalidate for the entry HTML (/ , *.html)
and sw.min.js so Vercel-served domains (and any CDN/browser in front) always
revalidate the shell and pick up new deploys, mirroring netlify.toml. Hashed
JS/CSS keep far-future caching via their content-hashed filenames.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

vercel.json

@@ -1,5 +1,25 @@
 {
   "buildCommand": "npm run build",
   "outputDirectory": "dist",
-  "installCommand": "npm install"
+  "installCommand": "npm install",
+  "headers": [
+    {
+      "source": "/sw.min.js",
+      "headers": [
+        { "key": "Cache-Control", "value": "no-cache, must-revalidate" }
+      ]
+    },
+    {
+      "source": "/",
+      "headers": [
+        { "key": "Cache-Control", "value": "no-cache, must-revalidate" }
+      ]
+    },
+    {
+      "source": "/(.*\\.html)",
+      "headers": [
+        { "key": "Cache-Control", "value": "no-cache, must-revalidate" }
+      ]
+    }
+  ]
 }