feat(server): embed SPA via memory-serve behind embed-web feature

Adds `memory-serve` 2.1 as an optional workspace dependency, a `build.rs`
that runs `load_directory` only when `CARGO_FEATURE_EMBED_WEB` is set, a
`web_assets` module serving `web/dist` at `/` with SPA fallback (200 OK)
for unknown client-side routes, and a feature-gated integration test.

The default build (no feature) compiles and tests cleanly without `web/dist`.

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

crates/server/tests/embed.rs

@@ -0,0 +1,37 @@
+//! Only meaningful with `--features embed-web` and a built `web/dist`. Compiled only
+//! under that feature.
+#![cfg(feature = "embed-web")]
+
+use axum::body::Body;
+use axum::http::{Request, StatusCode};
+use http_body_util::BodyExt;
+use tower::ServiceExt;
+
+#[tokio::test]
+async fn serves_index_at_root_and_spa_fallback() {
+    let app = server::test_support::web_router();
+
+    let root = app
+        .clone()
+        .oneshot(Request::builder().uri("/").body(Body::empty()).unwrap())
+        .await
+        .unwrap();
+
+    assert_eq!(root.status(), StatusCode::OK);
+
+    let body = root.into_body().collect().await.unwrap().to_bytes();
+
+    assert!(String::from_utf8_lossy(&body).contains("<div id=\"root\">"));
+
+    let deep = app
+        .oneshot(
+            Request::builder()
+                .uri("/objects/123")
+                .body(Body::empty())
+                .unwrap(),
+        )
+        .await
+        .unwrap();
+
+    assert_eq!(deep.status(), StatusCode::OK);
+}