feat: continue tree rust family cutover

- add rust renderer/state-family scaffolds and inline compat host thinning for page tree, file tree, and picker
- route tree/filetree preflight, file projection, resource artifact, and stream delta contracts through rust plans
- preserve canonical move-order validation, file-tree search projection, and related frontend/runtime regression coverage

rust/crates/mnote-web/src/error.rs

@@ -1,8 +1,8 @@
 use crate::context::RequestContext;
+use axum::Json;
 use axum::http::StatusCode;
 use axum::http::{HeaderName, HeaderValue};
 use axum::response::{IntoResponse, Response};
-use axum::Json;
 use serde::Serialize;
 
 #[derive(Debug, Clone, Serialize)]
@@ -68,6 +68,10 @@ impl WebError {
         self.headers.push((name, value.into()));
         self
     }
+
+    pub fn message(&self) -> &str {
+        &self.message
+    }
 }
 
 impl IntoResponse for WebError {