fix(web): center issue content and make width responsive

Use mx-auto with max-w-3xl so content stays centered in the
available space and scales with window width instead of being
pinned to the left with fixed padding.

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

apps/web/app/(dashboard)/issues/[id]/page.tsx

@@ -158,7 +158,7 @@ export default function IssueDetailPage({
         </div>
 
         {/* Content */}
-        <div className="px-12 py-8 max-w-[720px]">
+        <div className="mx-auto w-full max-w-3xl px-8 py-8">
           {/* Issue key */}
           <div className="mb-1 text-[13px] text-muted-foreground">{issue.key}</div>