feat(sidebar): read LibreChat URL from LIBRECHAT_URL env var

Replace hardcoded localhost:3080 chat link with configurable
LIBRECHAT_URL environment variable, passed through AuthInfo to
the sidebar component.

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

src/models/user.rs

@@ -22,6 +22,8 @@ pub struct AuthInfo {
     pub name: String,
     /// Avatar URL (from Keycloak picture claim)
     pub avatar_url: String,
+    /// LibreChat instance URL for the sidebar chat link
+    pub librechat_url: String,
 }
 
 /// Per-user LLM provider configuration stored in MongoDB.