Add a user guide, reachable in-app from Help in the account menu

USER_GUIDE.md at the repo root is the single source of truth --
frontend/public/USER_GUIDE.md symlinks to it so the same file is both
readable directly in the repo and served by the app, rendered on a new
/help page reusing the existing markdown renderer. Scoped to regular
member features (messaging, rooms, attachments, notifications,
profile); room admin/site admin features are intentionally left out.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/HelpPage.tsx

@@ -0,0 +1,55 @@
+import { useEffect, useState } from 'react'
+import Markdown from 'markdown-to-jsx'
+import { Link } from 'react-router-dom'
+import { MARKDOWN_OPTIONS } from '../components/MessageContent'
+import { TopBar } from '../components/TopBar'
+import './HelpPage.css'
+
+// Single source of truth is the repo-root USER_GUIDE.md -- frontend/public/
+// symlinks to it, so this fetches the same file a developer sees when
+// browsing the repo, rather than duplicating its content into the bundle.
+const GUIDE_URL = '/USER_GUIDE.md'
+
+export function HelpPage() {
+  const [content, setContent] = useState<string | null>(null)
+  const [error, setError] = useState<string | null>(null)
+
+  useEffect(() => {
+    let cancelled = false
+    fetch(GUIDE_URL)
+      .then((res) => {
+        if (!res.ok) throw new Error(`Failed to load guide (${res.status})`)
+        return res.text()
+      })
+      .then((text) => {
+        if (!cancelled) setContent(text)
+      })
+      .catch((err) => {
+        if (!cancelled) setError(err instanceof Error ? err.message : 'Failed to load guide')
+      })
+    return () => {
+      cancelled = true
+    }
+  }, [])
+
+  return (
+    <div className="help-page">
+      <TopBar />
+      <div className="help-body">
+        <div className="help-header">
+          <h1>Help</h1>
+          <Link to="/rooms" className="btn-secondary">
+            Back to chat
+          </Link>
+        </div>
+        {error && <p className="admin-error">{error}</p>}
+        {!error && content === null && <p className="help-loading">Loading…</p>}
+        {!error && content !== null && (
+          <div className="message-text help-content">
+            <Markdown options={MARKDOWN_OPTIONS}>{content}</Markdown>
+          </div>
+        )}
+      </div>
+    </div>
+  )
+}