feat(yahweh-widget-app-header): promover header standard de explorers

Iter 16. Patrón con 4 consumers idénticos: nakui-explorer,
nouser-explorer, minga-explorer, brahman-broker-explorer todos
declaraban un header flex_row + flex_grow(label) + theme_switcher
+ bg(panel) + border-bottom + text_size(14) + padding(16/12).
Ahora es 1 línea.

crates/modules/ui_engine/widgets/app-header/:
- pub fn app_header(cx, label: impl Into<SharedString>) -> impl IntoElement.
- pub fn app_header_with(cx, label_child: impl IntoElement) — variante
  para left side no-text.
- 3 tests #[gpui::test].

Migración 4 consumers:
- Cada uno: bloque de ~13 líneas → 1 línea app_header(cx, text).
- Borra dep yahweh-widget-theme-switcher (incluida vía app-header).
- Reemplaza import.

Ahorro ~50 líneas UI hardcoded. Cambios visuales del header (padding,
border, text_size) en un solo lugar.

Decisión: sidebar header del MetaApp NO se migra — es de sidebar,
no de app top, styling distinto. Diferente patrón → diferente widget.

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

crates/apps/minga-explorer/Cargo.toml

@@ -10,7 +10,7 @@ minga-store = { path = "../../modules/semantic_dht/minga-store" }
 yahweh-theme = { path = "../../modules/ui_engine/libs/theme" }
 yahweh-widget-banner = { path = "../../modules/ui_engine/widgets/banner" }
 yahweh-widget-stat-card = { path = "../../modules/ui_engine/widgets/stat-card" }
-yahweh-widget-theme-switcher = { path = "../../modules/ui_engine/widgets/theme-switcher" }
+yahweh-widget-app-header = { path = "../../modules/ui_engine/widgets/app-header" }
 gpui = { workspace = true }
 
 [[bin]]

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

@@ -34,7 +34,7 @@ use minga_store::PersistentRepo;
 use yahweh_theme::Theme;
 use yahweh_widget_banner::{banner_themed, Banner};
 use yahweh_widget_stat_card::stat_card;
-use yahweh_widget_theme_switcher::theme_switcher;
+use yahweh_widget_app_header::app_header;
 
 const REFRESH_INTERVAL: Duration = Duration::from_secs(2);
 const REPO_DIRNAME: &str = "repo";
@@ -224,19 +224,8 @@ impl Render for Explorer {
             None => format!("Buscando repo en {}…", self.repo_path.display()),
         };
 
-        let header = div()
-            .flex()
-            .flex_row()
-            .items_center()
-            .px(px(16.))
-            .py(px(12.))
-            .bg(theme.bg_panel.clone())
-            .border_b_1()
-            .border_color(theme.border)
-            .text_color(text)
-            .text_size(px(14.))
-            .child(div().flex_grow().child(header_text))
-            .child(theme_switcher(cx));
+        // Header standard via widget compartido.
+        let header = app_header(cx, header_text);
 
         let error_banner = self.error.as_ref().map(|e| {
             banner_themed(cx, Banner::Error, e.clone())