ui: command forms drawn from the parameter schema

The context panel's form shows typed fields instead of the JSON text: numbers
with the model's units, drop-downs of choices and of the model's sets,
materials, steps and bodies, checkboxes, vectors, groups and one-of variants
(a load's type switches its fields), lists of rows. Defaults are filled in,
required fields are marked, a value the schema finds wrong is noted under its
field, and OK waits while a field holds text that is not a value. The JSON
text stays as the Advanced view. fields.rs holds the generic widgets (row,
number, drop-down, text), adapted from SolidCraft's dialog rows.

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

crates/ui-egui/src/files.rs

@@ -1030,7 +1030,7 @@ mod tests {
         assert!(!app.display().bounds.is_empty());
 
         app.start("mesh.tet.generate").unwrap();
-        assert_eq!(app.dialog.as_ref().map(|d| (d.command, d.text.as_str())), Some(("mesh.tet.generate", "{}")));
+        assert_eq!(app.dialog.as_ref().map(|d| (d.command, d.text.as_str())), Some(("mesh.tet.generate", r#"{"order":"linear","quality":true}"#)));
         app.submit_dialog();
         assert_eq!(app.busy(), Some("Generate Tet Mesh"), "meshing runs off the window thread");
         app.wait_loaded();