feat(yahweh-widget-theme-switcher): control para ciclar themes en runtime

Iter 6. Cierra el ciclo del theme: ya teníamos paleta themed +
widgets que la consumen, faltaba el control UI para rotar entre
presets en vivo. Ahora hay un botón yahweh que muestra el theme
actual y al click avanza al siguiente.

crates/modules/ui_engine/widgets/theme-switcher/:
- pub fn theme_switcher(cx: &mut App) -> impl IntoElement: botón
  clickable con bg=panel_alt, hover=row_hover, label "Tema: <name> ▸".
  Al click hace Theme::set(cx, Theme::next_after(current.name)).
- 2 tests #[gpui::test]: smoke + verificación de cambio de global.
- Dev-dep gpui con test-support.

Migración:
- nakui-explorer: header pasa a flex_row con label flex_grow + switcher
  alineado derecha.
- yahweh-widget-meta-form (sidebar): mismo pattern en el header
  "Nakui" del sidebar.

Tests stack: 115 → 117.

Beneficio: click cambia toda la paleta en vivo. 6 presets disponibles
(Nebula, Aurora, Sunset, Flat Dark, Solarized Light, High Contrast)
ciclables circularmente.

Limitación: TextInput entities tienen colors hardcoded; migrar
text_input al theme es iter futura.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -6463,6 +6463,7 @@ dependencies = [
  "yahweh-theme",
  "yahweh-widget-banner",
  "yahweh-widget-card",
+ "yahweh-widget-theme-switcher",
 ]
 
 [[package]]
@@ -13037,6 +13038,7 @@ dependencies = [
  "yahweh-theme",
  "yahweh-widget-banner",
  "yahweh-widget-text-input",
+ "yahweh-widget-theme-switcher",
 ]
 
 [[package]]
@@ -13067,6 +13069,14 @@ dependencies = [
  "yahweh-theme",
 ]
 
+[[package]]
+name = "yahweh-widget-theme-switcher"
+version = "0.1.0"
+dependencies = [
+ "gpui",
+ "yahweh-theme",
+]
+
 [[package]]
 name = "yahweh-widget-tiled"
 version = "0.1.0"