fix(ai-advice,ui): Bearer auth for proxy gateways + un-pin contact card + VN diacritics

- AI advice handler now sends both `x-api-key` and `Authorization: Bearer`
  so proxy gateways (e.g. chat.trollllm.xyz) accept the request. Native
  Anthropic ignores the extra header.
- Remove `lg:sticky lg:top-20` from listing detail contact card — sidebar
  now scrolls with the page.
- Fix missing Vietnamese diacritics on AI estimate button:
  "Dinh gia AI" -> "Định giá AI", "Dang dinh gia..." -> "Đang định giá...".
  Tests updated accordingly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/components/listings/listing-detail-client.tsx

@@ -375,7 +375,7 @@ export function ListingDetailClient({ listing }: ListingDetailClientProps) {
         {/* Sidebar */}
         <div className="space-y-6">
           {/* Contact card */}
-          <Card className="lg:sticky lg:top-20">
+          <Card>
             <CardHeader>
               <CardTitle>Liên hệ</CardTitle>
             </CardHeader>