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>

Cargo.lock

@@ -1243,9 +1243,9 @@ dependencies = [
  "brahman-sidecar",
  "gpui",
  "yahweh-theme",
+ "yahweh-widget-app-header",
  "yahweh-widget-banner",
  "yahweh-widget-stat-card",
- "yahweh-widget-theme-switcher",
 ]
 
 [[package]]
@@ -6216,9 +6216,9 @@ dependencies = [
  "gpui",
  "minga-store",
  "yahweh-theme",
+ "yahweh-widget-app-header",
  "yahweh-widget-banner",
  "yahweh-widget-stat-card",
- "yahweh-widget-theme-switcher",
 ]
 
 [[package]]
@@ -6486,9 +6486,9 @@ dependencies = [
  "uuid",
  "yahweh-meta-runtime",
  "yahweh-theme",
+ "yahweh-widget-app-header",
  "yahweh-widget-banner",
  "yahweh-widget-card",
- "yahweh-widget-theme-switcher",
 ]
 
 [[package]]
@@ -6883,9 +6883,9 @@ dependencies = [
  "gpui",
  "nouser-card",
  "yahweh-theme",
+ "yahweh-widget-app-header",
  "yahweh-widget-banner",
  "yahweh-widget-card",
- "yahweh-widget-theme-switcher",
 ]
 
 [[package]]
@@ -13031,6 +13031,15 @@ dependencies = [
  "gpui",
 ]
 
+[[package]]
+name = "yahweh-widget-app-header"
+version = "0.1.0"
+dependencies = [
+ "gpui",
+ "yahweh-theme",
+ "yahweh-widget-theme-switcher",
+]
+
 [[package]]
 name = "yahweh-widget-banner"
 version = "0.1.0"