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/components/TopBar.tsx

@@ -122,6 +122,16 @@ export function TopBar() {
               >
                 Profile settings
               </button>
+              <button
+                type="button"
+                role="menuitem"
+                onClick={() => {
+                  setMenuOpen(false)
+                  navigate('/help')
+                }}
+              >
+                Help
+              </button>
               <button
                 type="button"
                 role="menuitem"