Format display tag and username with prefix symbols for consistency in Open Graph API responses

Co-authored-by: Copilot <copilot@github.com>

src/app/api/og/hashtag/[tag]/route.tsx

@@ -87,7 +87,7 @@ export async function GET(
           <div style={{ display: 'flex', flexDirection: 'column' }}>
             <div style={{ fontSize: 28, color: '#6366f1', marginBottom: 8 }}>HashEx</div>
             <div style={{ fontSize: 72, fontWeight: 700, color: '#ffffff' }}>
-              #{displayTag}
+              {'#' + displayTag}
             </div>
           </div>
           <div style={{ display: 'flex', flexDirection: 'column', alignItems: 'flex-end' }}>

src/app/api/og/profile/[username]/route.tsx

@@ -57,7 +57,7 @@ export async function GET(
 
         {/* Name */}
         <div style={{ fontSize: 68, fontWeight: 700, color: '#ffffff', marginBottom: 48 }}>
-          @{displayName}
+          {'@' + displayName}
         </div>
 
         {/* Stats grid */}