UI: a reference field is always a drop-down; with nothing to pick it offers Create …, and one candidate starts picked (#215)

A reference field (a material, set, step…) with no items was a free-text box; it is now a greyed
drop-down ("No materials yet") with a button that opens the command making one (the
collection's requirement provider). That form comes back to this one on OK, with the new item
picked, or as it was on Cancel. A required reference with one candidate, or with the browser's
selection of its kind, starts picked when the form opens, so the fields and the JSON view agree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/context.rs

@@ -118,6 +118,7 @@ fn form(app: &mut ModelApp, ui: &mut egui::Ui, t: &Tokens) {
         }
     });
     let mut invalid = 0;
+    let mut create = None;
     // The typed fields edit the JSON text's object; text that is not an object stays in the
     // JSON view until it is.
     let object = params.as_object().cloned();
@@ -135,6 +136,7 @@ fn form(app: &mut ModelApp, ui: &mut egui::Ui, t: &Tokens) {
             let cx = crate::form::Context { units, names: &|c| session.names(c), salt: form_id };
             let out = crate::form::show(ui, t, &cx, fields, &mut obj);
             invalid = out.invalid;
+            create = out.create;
             if out.changed {
                 d.text = Value::Object(obj).to_string();
                 d.error = None;
@@ -167,7 +169,9 @@ fn form(app: &mut ModelApp, ui: &mut egui::Ui, t: &Tokens) {
         cancel = true;
     }
     if cancel {
-        app.dialog = None;
+        app.close_dialog();
+    } else if let Some((collection, path)) = create {
+        app.create_from_form(collection, path);
     } else if run {
         app.submit_dialog();
     } else if use_selection