fix(routes): clearing observers in the edit modal now persists

The route edit modal builds its PUT body with a ternary that collapses
an empty observer list to null:

    observer_public_keys: observerPublicKeys.length > 0 ? observerPublicKeys : null,

Pydantic parses null as None, and the PUT handler's guard

    if body.observer_public_keys is not None:
        _sync_observers(session, route, observer_nodes)

skips the sync entirely when the field is None. So removing all
observers in the modal sent null -> no DB change. Adding observers
worked because a non-empty array passed the guard and _sync_observers
deleted + recreated.

The None-means-skip semantic is correct for true partial updates, so
the fix is on the frontend: the edit modal is a full-form PUT, so it
must always send the array. When empty, it sends [], which Pydantic
parses as [] (not None), the guard passes, and _sync_observers deletes
every existing RouteObserver row.

Tests: add test_update_clear_observers_with_empty_list next to the
existing test_update_observers as a regression guard. It seeds one
observer via PUT, then PUTs observer_public_keys: [] and asserts both
the response and a fresh GET come back with an empty list.

src/meshcore_hub/web/static/js/spa/pages/routes.js

@@ -809,7 +809,7 @@ export async function render(container, params, router) {
                 enabled: enabledEl.checked,
                 reversible: reversibleEl.checked,
                 node_public_keys: nodePublicKeys,
-                observer_public_keys: observerPublicKeys.length > 0 ? observerPublicKeys : null,
+                observer_public_keys: observerPublicKeys,
             };
 
             const clearVal = clearEl.value.trim();

tests/test_api/test_routes.py

@@ -995,6 +995,42 @@ class TestUpdateRouteFields:
         assert len(data["route_observers"]) == 1
         assert data["route_observers"][0]["public_key"] == obs.public_key
 
+    def test_update_clear_observers_with_empty_list(
+        self, client_no_auth, api_db_session
+    ):
+        """Sending ``observer_public_keys: []`` must clear existing observers.
+
+        Regression guard for a frontend bug where the edit modal sent
+        ``null`` instead of ``[]`` when the user removed all observers,
+        causing the backend's ``is not None`` guard to skip the sync.
+        """
+        route = self._make_route(api_db_session)
+        obs = _make_node(api_db_session, "c" * 64, "Observer")
+        api_db_session.commit()
+
+        # Seed one observer.
+        client_no_auth.put(
+            f"/api/v1/routes/{route.id}",
+            json={"observer_public_keys": [obs.public_key]},
+            headers={"X-User-Roles": "admin"},
+        )
+
+        # Clear with an explicit empty list.
+        resp = client_no_auth.put(
+            f"/api/v1/routes/{route.id}",
+            json={"observer_public_keys": []},
+            headers={"X-User-Roles": "admin"},
+        )
+        assert resp.status_code == 200
+        assert resp.json()["route_observers"] == []
+
+        # Confirm persistence via a fresh GET.
+        detail = client_no_auth.get(
+            f"/api/v1/routes/{route.id}",
+            headers={"X-User-Roles": "admin"},
+        ).json()
+        assert detail["route_observers"] == []
+
     def test_update_unresolved_path_nodes_400(self, client_no_auth, api_db_session):
         route = self._make_route(api_db_session)
         api_db_session.commit()