frontend: add bookmark management and homepage navigation

Admin-only /bookmarks page for managing entries; homepage now renders
public bookmarks as a category-grouped navigation grid (empty state
links admin to the manager). Dashboard gains a recent-bookmarks card,
dock and main layout get a bookmark entry for admins, and the
middleware protects /bookmarks.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/middleware.ts

@@ -11,6 +11,7 @@ export default auth((req) => {
                      nextUrl.pathname.startsWith("/bind-account");
 
   const isProtected = nextUrl.pathname.startsWith("/dashboard") ||
+                      nextUrl.pathname.startsWith("/bookmarks") ||
                       nextUrl.pathname.startsWith("/admin");
 
   if (isLoggedIn && isAuthPage) {