feat(developer): replace agents iframe with informational landing and live agent table

LangGraph is API-only with no web UI, so the ToolEmbed iframe pattern
doesn't work. Replace it with an informational landing page featuring a
hero section, connection status indicator, quick-start card grid linking
to docs/GitHub/examples, and a live table of registered agents fetched
from the LangGraph POST /assistants/search endpoint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/infrastructure/mod.rs

@@ -2,6 +2,7 @@
 // the #[server] macro generates client stubs for the web target)
 pub mod auth_check;
 pub mod chat;
+pub mod langgraph;
 pub mod llm;
 pub mod ollama;
 pub mod searxng;