feat(yahweh-widget-banner): widget compartido para toasts/errores cross-app

Patrón visual común a yahweh-widget-meta-form (toast success +
error banner) y nakui-explorer (error banner): div con bg + text
colored según severidad. Antes duplicado con colores hardcoded en
cada consumer.

Crate nuevo crates/modules/ui_engine/widgets/banner:
- pub enum Banner { Info, Success, Warning, Error } con bg()/fg()
  hardcoded por variant.
- pub fn banner(kind, message) -> Div: padding/text_size defaults;
  caller compone con .child()/.px()/.on_click()/etc.
- 2 tests sanity (no color collisions).

Migración:
- yahweh-widget-meta-form: 12 líneas hardcoded → 2 llamadas a
  banner().
- nakui-explorer: error banner usa banner() + override de
  padding custom del header.

Tests stack: 109 → 111 (+2). Cada crate compila individualmente.

Próximo natural: confirm_delete_banner (Warning + botones) puede
extraerse como modal-banner cuando emerja segundo consumer.

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

crates/modules/ui_engine/widgets/meta-form/Cargo.toml

@@ -12,6 +12,7 @@ uuid = { workspace = true, features = ["serde"] }
 yahweh-meta-runtime = { path = "../../libs/meta-runtime" }
 yahweh-meta-schema = { path = "../../libs/meta-schema" }
 yahweh-theme = { path = "../../libs/theme" }
+yahweh-widget-banner = { path = "../banner" }
 yahweh-widget-text-input = { path = "../text_input" }
 
 [dev-dependencies]

crates/modules/ui_engine/widgets/meta-form/src/lib.rs

@@ -34,6 +34,7 @@ use yahweh_meta_runtime::{
     MetaBackend, WriteOutcome,
 };
 use yahweh_meta_schema::{Action, FieldKind, FieldSpec, FormView, ListView, Module, View};
+use yahweh_widget_banner::{banner, Banner};
 use yahweh_widget_text_input::TextInput;
 
 /// Estado del runtime de UI. Toda la persistencia/ejecución está
@@ -450,24 +451,14 @@ impl<B: MetaBackend> Render for MetaApp<B> {
         let sidebar = self.render_sidebar(cx, panel, border, text, text_dim, accent_active);
         let main_panel = self.render_main(cx, panel, border, text, text_dim, accent);
         let confirm_banner = self.render_confirm_delete_banner(cx);
-        let toast_div = self.toast.as_ref().map(|t| {
-            div()
-                .px(px(12.))
-                .py(px(6.))
-                .bg(gpui::rgb(0x2d3a2a))
-                .text_color(gpui::rgb(0xc0e0a0))
-                .text_size(px(11.))
-                .child(t.clone())
-        });
-        let error_banner = self.load_error.as_ref().map(|e| {
-            div()
-                .px(px(12.))
-                .py(px(6.))
-                .bg(gpui::rgb(0x4a2020))
-                .text_color(gpui::rgb(0xffd0d0))
-                .text_size(px(11.))
-                .child(e.clone())
-        });
+        let toast_div = self
+            .toast
+            .as_ref()
+            .map(|t| banner(Banner::Success, t.clone()));
+        let error_banner = self
+            .load_error
+            .as_ref()
+            .map(|e| banner(Banner::Error, e.clone()));
 
         div()
             .flex()