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>

crates/apps/nakui-explorer/Cargo.toml

@@ -11,6 +11,7 @@ yahweh-meta-runtime = { path = "../../modules/ui_engine/libs/meta-runtime" }
 yahweh-widget-banner = { path = "../../modules/ui_engine/widgets/banner" }
 yahweh-widget-card = { path = "../../modules/ui_engine/widgets/card" }
 yahweh-theme = { path = "../../modules/ui_engine/libs/theme" }
+yahweh-widget-theme-switcher = { path = "../../modules/ui_engine/widgets/theme-switcher" }
 gpui = { workspace = true }
 serde_json = { workspace = true }
 uuid = { workspace = true, features = ["serde"] }

crates/apps/nakui-explorer/src/main.rs

@@ -34,6 +34,7 @@ use yahweh_meta_runtime::{preview_value, short_hash, short_uuid};
 use yahweh_theme::Theme;
 use yahweh_widget_banner::{banner_themed, Banner};
 use yahweh_widget_card::card_themed;
+use yahweh_widget_theme_switcher::theme_switcher;
 
 const REFRESH_INTERVAL: Duration = Duration::from_secs(2);
 
@@ -167,7 +168,13 @@ impl Render for Explorer {
             self.last_load_ms,
         );
 
+        // Header con título a la izquierda + theme switcher a la
+        // derecha. flex_row + flex_grow del label empuja el switcher
+        // al borde.
         let header = div()
+            .flex()
+            .flex_row()
+            .items_center()
             .px(px(16.))
             .py(px(12.))
             .bg(theme.bg_panel.clone())
@@ -175,7 +182,8 @@ impl Render for Explorer {
             .border_color(theme.border)
             .text_color(text)
             .text_size(px(14.))
-            .child(header_text);
+            .child(div().flex_grow().child(header_text))
+            .child(theme_switcher(cx));
 
         let breakdown_line = if top_breakdown.is_empty() {
             String::new()