chore: change style of unknown OS download button

When we can determine the user's OS, we have a nice purple Download
Now button on the front page.
When we can't determine the user's OS, it's a full-width blue
button linking to the Download Page instead. This button doesn't
match our current styling at all.

I copied from the Download Now button to make it match:
- Added outer div (makes it normal size and centered).
- Changed indigo to purple.
- Added 'mt-6 mb-1' to space it from above and below.

Signed-off-by: Tim deBoer <git@tdeboer.ca>

website/src/pages/index.tsx

@@ -67,11 +67,13 @@ function DownloadClientLinks() {
     );
   } else {
     mainButton = (
-      <Link
-        className="no-underline hover:no-underline inline-flex text-white hover:text-white bg-indigo-500 border-0 py-2 px-6 focus:outline-none hover:bg-indigo-600 rounded text-lg"
-        to="/downloads">
-        Download Page
-      </Link>
+      <div>
+        <Link
+          className="no-underline hover:no-underline inline-flex text-white hover:text-white bg-purple-500 border-0 py-2 px-6 mt-6 mb-1 focus:outline-none hover:bg-purple-600 rounded text-lg"
+          to="/downloads">
+          Download Page
+        </Link>
+      </div>
     );
   }