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/save_changes.rs

@@ -0,0 +1,19 @@
+//! Save Changes (closing a modified document): Save / Don't Save / Cancel. The flow lives in
+//! [`crate::unsaved`]; this is its face in the shared dialog frame.
+
+use super::DialogSpec;
+
+pub(super) const SPEC: DialogSpec = DialogSpec {
+    heading: |d| format!("Do you want to save the changes you made to “{}”?", d.str("name")),
+    body: |_, ui, _| {
+        let t = crate::theme::Tokens::get(ui.ctx());
+        ui.label(egui::RichText::new("Your changes will be lost if you don't save them.").color(t.text_dim));
+        false
+    },
+    confirm: |app, _| crate::unsaved::confirm(app),
+    ok: Some("Save"),
+    discard: Some("Don't Save"),
+    // Long document names wrap instead of widening the dialog.
+    max_width: Some(420.0),
+    ..DialogSpec::FORM
+};