chat: tell users in the welcome line that their key is already connected (co-op/support#40)

The key is auto-seeded within 10 minutes of first login by sync-keys.js, but a new user's first screen is the API-keys panel, which reads as mandatory. The welcome line is the surface we control; the settings dialog builds its strings from templates in a component chunk with no en locale bundle, so it is not cleanly patchable. Option 1 of three in the issue, approved by the GM.

librechat.yaml

@@ -91,7 +91,7 @@ modelSpecs:
 # Inference Cooperative branding
 
 interface:
-  customWelcome: "Welcome to your co-op AI."
+  customWelcome: "Welcome to your co-op AI. Your account is already connected to the co-op's gateway, so there is no API key to paste."
   # Tools pinned to the composer bar by default (members can unpin).
   # NOTE: 'mcp' MUST be listed or the Tools button itself gets unpinned by
   # the client's default-pinning seed (hooks chunk gates PIN_MCP_ on this).