fix: remove hardcoded text-sm from media placeholder in Link.tsx (#30)

The MediaPlaceholder component in Link.tsx had text-sm hardcoded,
causing it to appear larger than surrounding text in compact contexts.
Now all placeholder components (Link, Gallery, Mention) consistently
inherit font size from their parent.

Co-authored-by: Claude <noreply@anthropic.com>

src/components/nostr/RichText/Link.tsx

@@ -10,7 +10,7 @@ import { PlainLink } from "../LinkPreview";
 import { useRichTextOptions } from "../RichText";
 
 function MediaPlaceholder({ type }: { type: "image" | "video" | "audio" }) {
-  return <span className="text-muted-foreground text-sm">[{type}]</span>;
+  return <span className="text-muted-foreground">[{type}]</span>;
 }
 
 interface LinkNodeProps {