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/engine/src/tests_panelcmds.rs

@@ -1,4 +1,4 @@
-//! Panel-support commands (`cmd/panelcmds.rs`).
+//! Panel-support commands (`cmd/{panelcmds,gradient,appearance,stroke,swatch,style}.rs`).
 
 use serde_json::json;
 use vectorcraft_color::{GradientKind, Paint};