M3.8: split the paint/panel command modules and the dialogs into per-area files

No behaviour change: every command keeps its id, label, menu, shortcut and
params doc.

- Engine: swatch.* -> cmd/swatch.rs, graphicStyle.* -> cmd/style.rs,
  appearance.* (incl. copyFrom) -> cmd/appearance.rs, stroke.set/setAdvanced
  -> cmd/stroke.rs, paint.editGradient/setGradientGeom -> cmd/gradient.rs.
  Shared helpers in cmd/mod.rs: leaf_targets/paint_targets (replacing three
  copies) and unique_name (swatch and graphic style naming).
- UI: dialogs.rs -> dialogs/ with one file per dialog and a DialogSpec
  registry (one DialogKind row per dialog); shared field widgets in
  dialogs/form.rs. Headless frame tests draw and confirm every dialog.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/dialogs/command.rs

@@ -0,0 +1,26 @@
+//! The generic parameter dialog (`ui.paramDialog`): edits a command's parameters (`__command`,
+//! headed `__label`) and runs it on OK.
+
+use serde_json::Value;
+
+use super::{DialogSpec, form};
+use crate::VectorcraftApp;
+use crate::state::Dialog;
+
+pub(super) const SPEC: DialogSpec = DialogSpec {
+    heading: |d| d.str("__label"),
+    body: |_, ui, d| {
+        form::param_fields(ui, d);
+        false
+    },
+    confirm,
+    ..DialogSpec::FORM
+};
+
+/// Closes before running, so a dialog the command opens stays open.
+fn confirm(app: &mut VectorcraftApp, d: &Dialog) -> Result<Value, String> {
+    let cmd = d.str("__command");
+    let params = form::params(d);
+    app.ui.dialog = None;
+    app.run(&cmd, params)
+}