feat: linkify resolves '<platform>: @handle' social mentions

Platform-prefixed handles (x/twitter/instagram/facebook/tiktok/youtube/twitch/threads/
telegram/soundcloud) now link to the platform's profile URL — only the @handle is
linked, the '<platform>:' label stays as text. A required colon/arrow separator + the
known-platform list keep false positives out; a bare @handle with no platform stays plain.

frontend/src/lib/linkify.tsx

@@ -1,43 +1,73 @@
 import { Fragment, type ReactNode } from "react";
 
-// Recognises three link shapes in plain text:
-//   1. explicit http(s):// URLs
-//   2. www.* bare hosts
-//   3. domain.tld/path bare URLs — the required path AND a letters-only TLD keep false positives
-//      (version numbers like "3.2/x", file names like "index.html") from matching.
-// The last matched char excludes trailing sentence punctuation, so "see foo.com/bar." links
-// "foo.com/bar" and leaves the period. Bare @handles are intentionally NOT linked — the platform
-// is ambiguous.
-const URL_RE =
-  /((?:https?:\/\/|www\.)[^\s<]+[^\s<.,!?;:)\]}"']|[a-z0-9-]+(?:\.[a-z0-9-]+)*\.[a-z]{2,}\/[^\s<]*[^\s<.,!?;:)\]}"'])/gi;
+// Known platform → profile base URL, for "<platform>: @handle" mentions in descriptions.
+const PLATFORMS: Record<string, string> = {
+  x: "https://x.com/",
+  twitter: "https://x.com/",
+  instagram: "https://instagram.com/",
+  insta: "https://instagram.com/",
+  ig: "https://instagram.com/",
+  facebook: "https://facebook.com/",
+  fb: "https://facebook.com/",
+  tiktok: "https://www.tiktok.com/@",
+  youtube: "https://www.youtube.com/@",
+  yt: "https://www.youtube.com/@",
+  twitch: "https://www.twitch.tv/",
+  threads: "https://www.threads.net/@",
+  telegram: "https://t.me/",
+  soundcloud: "https://soundcloud.com/",
+};
+
+// Two link shapes, combined into one scanner:
+//   URL — explicit http(s):// URLs, www.* hosts, or domain.tld/path bare URLs (required path +
+//         letters-only TLD keep version numbers / file names out). Non-http gets an https:// href.
+//   MENTION — "<platform>: @handle" where <platform> is one of the known keys above. The colon/arrow
+//         separator is REQUIRED, so a stray "x" in prose won't match. Only the @handle is linked
+//         (to the platform's profile URL); the "<platform>: " label stays as text. Bare @handles
+//         with no platform, and #hashtags, are left plain.
+const URL_SOURCE =
+  '(?:https?://|www\\.)[^\\s<]+[^\\s<.,!?;:)\\]}"\']|[a-z0-9-]+(?:\\.[a-z0-9-]+)*\\.[a-z]{2,}/[^\\s<]*[^\\s<.,!?;:)\\]}"\']';
+const MENTION_SOURCE =
+  "\\b(" +
+  Object.keys(PLATFORMS).join("|") +
+  ")\\b[ \\t]*[:：►▶▸]+[ \\t]*@([a-z0-9_](?:[a-z0-9_.]{0,28}[a-z0-9_])?)";
+const TOKEN_RE = new RegExp("(" + URL_SOURCE + ")|" + MENTION_SOURCE, "gi");
 
 /**
- * Turn bare URLs (http(s), www, or domain.tld/path) inside a plain-text string into links that open
- * in a new tab (safe rel). Non-http matches get an https:// prefix on the href, but keep their
- * original text. Everything else is returned as text, so it composes with `whitespace-pre-wrap`.
+ * Turn links inside a plain-text string into clickable <a>s that open in a new tab (safe rel):
+ * bare URLs (http(s)/www/domain.tld/path) and "<platform>: @handle" social mentions. Everything
+ * else is returned as text, so it composes with `whitespace-pre-wrap` (line breaks preserved).
  */
 export function linkify(text: string): ReactNode[] {
   const out: ReactNode[] = [];
   let last = 0;
   let key = 0;
-  URL_RE.lastIndex = 0;
+  TOKEN_RE.lastIndex = 0;
+  const link = (href: string, label: string) => (
+    <a
+      key={key++}
+      href={href}
+      target="_blank"
+      rel="noopener noreferrer"
+      className="text-accent hover:underline break-all"
+    >
+      {label}
+    </a>
+  );
   let m: RegExpExecArray | null;
-  while ((m = URL_RE.exec(text)) !== null) {
+  while ((m = TOKEN_RE.exec(text)) !== null) {
     if (m.index > last) out.push(<Fragment key={key++}>{text.slice(last, m.index)}</Fragment>);
-    const raw = m[0];
-    const href = /^https?:\/\//i.test(raw) ? raw : `https://${raw}`;
-    out.push(
-      <a
-        key={key++}
-        href={href}
-        target="_blank"
-        rel="noopener noreferrer"
-        className="text-accent hover:underline break-all"
-      >
-        {raw}
-      </a>,
-    );
-    last = m.index + raw.length;
+    if (m[1]) {
+      // Plain URL.
+      const raw = m[1];
+      out.push(link(/^https?:\/\//i.test(raw) ? raw : `https://${raw}`, raw));
+    } else {
+      // "<platform>: @handle" — keep the "<platform>: " prefix as text, link only the handle.
+      const handle = m[3];
+      out.push(<Fragment key={key++}>{m[0].slice(0, m[0].length - handle.length - 1)}</Fragment>);
+      out.push(link(PLATFORMS[m[2].toLowerCase()] + handle, "@" + handle));
+    }
+    last = m.index + m[0].length;
   }
   if (last < text.length) out.push(<Fragment key={key++}>{text.slice(last)}</Fragment>);
   return out;