Glue icon to link text, add external-link indicator

PAP-2240: the GitHub icon on github.com markdown links could wrap onto
the previous line, separated from the first character of the URL. Pin
the icon and first character inside a white-space:nowrap span so they
never split across a line break. Also add an ExternalLink (lucide) icon
on the right of any target="_blank" link, glued to the last character
with the same no-wrap treatment so the "opens in new tab" cue stays
with the link text.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/MarkdownBody.test.tsx

@@ -316,12 +316,15 @@ describe("MarkdownBody", () => {
     expect(html).toContain('rel="noreferrer"');
   });
 
-  it("prefixes GitHub markdown links with the GitHub icon", () => {
+  it("prefixes GitHub markdown links with the GitHub icon glued to the first character", () => {
     const html = renderMarkdown("[https://github.com/paperclipai/paperclip/pull/4099](https://github.com/paperclipai/paperclip/pull/4099)");
 
     expect(html).toContain('<a href="https://github.com/paperclipai/paperclip/pull/4099"');
     expect(html).toContain('class="lucide lucide-github mr-1 inline h-3.5 w-3.5 align-[-0.125em]"');
-    expect(html).toContain(">https://github.com/paperclipai/paperclip/pull/4099</a>");
+    // The icon and first character "h" must sit in a no-wrap span so the
+    // icon can never be orphaned on the previous line from the URL text.
+    expect(html).toMatch(/<span style="white-space:nowrap">.*lucide-github.*?<\/svg>h<\/span>/);
+    expect(html).toContain("ttps://github.com/paperclipai/paperclip/pull/4099");
   });
 
   it("prefixes GitHub autolinks with the GitHub icon", () => {
@@ -338,6 +341,22 @@ describe("MarkdownBody", () => {
     expect(html).not.toContain("lucide-github");
   });
 
+  it("suffixes external links with a new-tab icon glued to the last character", () => {
+    const html = renderMarkdown("[docs](https://example.com/docs)");
+
+    expect(html).toContain('target="_blank"');
+    expect(html).toContain("lucide-external-link");
+    // Last character "s" must sit in a no-wrap span with the icon so the
+    // indicator never wraps away from the link text.
+    expect(html).toMatch(/<span style="white-space:nowrap">s<svg[^>]*lucide-external-link/);
+  });
+
+  it("does not render the new-tab icon on internal links", () => {
+    const html = renderMarkdown("[settings](/company/settings)");
+
+    expect(html).not.toContain("lucide-external-link");
+  });
+
   it("keeps fenced code blocks width-bounded and horizontally scrollable", () => {
     const html = renderMarkdown("```text\nGET /heartbeat-runs/ca5d23fc-c15b-4826-8ff1-2b6dd11be096/log?offset=2062357&limitBytes=256000\n```");
 

ui/src/components/MarkdownBody.tsx

@@ -1,6 +1,6 @@
 import { isValidElement, useEffect, useId, useState, type ReactNode } from "react";
 import { useQuery } from "@tanstack/react-query";
-import { Github } from "lucide-react";
+import { ExternalLink, Github } from "lucide-react";
 import Markdown, { defaultUrlTransform, type Components, type Options } from "react-markdown";
 import remarkGfm from "remark-gfm";
 import { cn } from "../lib/utils";
@@ -133,6 +133,54 @@ function isExternalHttpUrl(href: string | null | undefined): boolean {
   }
 }
 
+function renderLinkBody(
+  children: ReactNode,
+  leadingIcon: ReactNode,
+  trailingIcon: ReactNode,
+): ReactNode {
+  if (!leadingIcon && !trailingIcon) return children;
+
+  if (typeof children === "string" && children.length > 0) {
+    if (children.length === 1) {
+      return (
+        <span style={{ whiteSpace: "nowrap" }}>
+          {leadingIcon}
+          {children}
+          {trailingIcon}
+        </span>
+      );
+    }
+    const first = children[0];
+    const last = children[children.length - 1];
+    const middle = children.slice(1, -1);
+    return (
+      <>
+        {leadingIcon ? (
+          <span style={{ whiteSpace: "nowrap" }}>
+            {leadingIcon}
+            {first}
+          </span>
+        ) : first}
+        {middle}
+        {trailingIcon ? (
+          <span style={{ whiteSpace: "nowrap" }}>
+            {last}
+            {trailingIcon}
+          </span>
+        ) : last}
+      </>
+    );
+  }
+
+  return (
+    <>
+      {leadingIcon}
+      {children}
+      {trailingIcon}
+    </>
+  );
+}
+
 function MermaidDiagramBlock({ source, darkMode }: { source: string; darkMode: boolean }) {
   const renderId = useId().replace(/[^a-zA-Z0-9_-]/g, "");
   const [svg, setSvg] = useState<string | null>(null);
@@ -281,6 +329,12 @@ export function MarkdownBody({
       }
       const isGitHubLink = isGitHubUrl(href);
       const isExternal = isExternalHttpUrl(href);
+      const leadingIcon = isGitHubLink ? (
+        <Github aria-hidden="true" className="mr-1 inline h-3.5 w-3.5 align-[-0.125em]" />
+      ) : null;
+      const trailingIcon = isExternal ? (
+        <ExternalLink aria-hidden="true" className="ml-1 inline h-3 w-3 align-[-0.125em]" />
+      ) : null;
       return (
         <a
           href={href}
@@ -289,8 +343,7 @@ export function MarkdownBody({
             : { rel: "noreferrer" })}
           style={mergeWrapStyle(linkStyle as React.CSSProperties | undefined)}
         >
-          {isGitHubLink ? <Github aria-hidden="true" className="mr-1 inline h-3.5 w-3.5 align-[-0.125em]" /> : null}
-          {linkChildren}
+          {renderLinkBody(linkChildren, leadingIcon, trailingIcon)}
         </a>
       );
     },