theme config, layout preview, container alignment

Server: ThemeConfig entity + CRUD (GET/PUT /theme), SQLite persistence,
ThemeUpdate broadcast to ESP32 on save and initial connect.
Client: render engine uses theme colors, full-screen redraw on theme change.
SPA: theme page with color pickers + presets, layout preview with TS port
of layout engine, justify/align controls on containers.
DisplayHint refactored to struct (kind + h_align + v_align).

crates/domain/src/ports/broadcast.rs

@@ -1,5 +1,5 @@
 use crate::entities::WidgetId;
-use crate::value_objects::{DisplayHint, Layout, WidgetState};
+use crate::value_objects::{DisplayHint, Layout, ThemeConfig, WidgetState};
 use std::future::Future;
 
 pub trait BroadcastPort {
@@ -15,4 +15,9 @@ pub trait BroadcastPort {
         &self,
         updates: &[(WidgetId, DisplayHint, WidgetState)],
     ) -> impl Future<Output = Result<(), Self::Error>> + Send;
+
+    fn push_theme_update(
+        &self,
+        theme: &ThemeConfig,
+    ) -> impl Future<Output = Result<(), Self::Error>> + Send;
 }