fix(markdown): allow mention:// protocol through URL sanitization

react-markdown v10's defaultUrlTransform strips URLs with non-standard
protocols (only http/https/irc/mailto/xmpp allowed). This caused
mention://issue/ links to have empty hrefs, breaking click navigation
to issue detail pages.

apps/web/components/markdown/Markdown.tsx

@@ -1,6 +1,6 @@
 import * as React from 'react'
 import Link from 'next/link'
-import ReactMarkdown, { type Components } from 'react-markdown'
+import ReactMarkdown, { type Components, defaultUrlTransform } from 'react-markdown'
 import rehypeRaw from 'rehype-raw'
 import remarkGfm from 'remark-gfm'
 import { cn } from '@/lib/utils'
@@ -44,6 +44,15 @@ export interface MarkdownProps {
   onFileClick?: (path: string) => void
 }
 
+/**
+ * Custom URL transform that allows mention:// protocol (used for @mentions)
+ * while keeping the default security for all other URLs.
+ */
+function urlTransform(url: string): string {
+  if (url.startsWith('mention://')) return url
+  return defaultUrlTransform(url)
+}
+
 // File path detection regex - matches paths starting with /, ~/, or ./
 const FILE_PATH_REGEX =
   /^(?:\/|~\/|\.\/)[\w\-./@]+\.(?:ts|tsx|js|jsx|mjs|cjs|md|json|yaml|yml|py|go|rs|css|scss|less|html|htm|txt|log|sh|bash|zsh|swift|kt|java|c|cpp|h|hpp|rb|php|xml|toml|ini|cfg|conf|env|sql|graphql|vue|svelte|astro|prisma)$/i
@@ -298,6 +307,7 @@ export function Markdown({
       <ReactMarkdown
         remarkPlugins={[[remarkGfm, { singleTilde: false }]]}
         rehypePlugins={[rehypeRaw]}
+        urlTransform={urlTransform}
         components={components}
       >
         {processedContent}