feat: add system configuration API endpoint and frontend hook for dynamic settings

notes-api/src/routes/config.rs

@@ -0,0 +1,14 @@
+//! Configuration routes
+
+use axum::{Json, extract::State};
+
+use crate::dto::ConfigResponse;
+use crate::error::ApiResult;
+use crate::state::AppState;
+
+/// Get system configuration
+pub async fn get_config(State(state): State<AppState>) -> ApiResult<Json<ConfigResponse>> {
+    Ok(Json(ConfigResponse {
+        allow_registration: state.config.allow_registration,
+    }))
+}