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/bootstrap/src/event_handler.rs

@@ -40,6 +40,12 @@ pub async fn run(
 
                 info!("layout changed, pushed screen update to clients");
             }
+            Ok(DomainEvent::ThemeChanged { theme }) => {
+                if let Err(e) = broadcaster.push_theme_update(&theme).await {
+                    error!(error = %e, "failed to push theme update");
+                }
+                info!("theme changed, pushed update to clients");
+            }
             Ok(_) => {}
             Err(tokio::sync::broadcast::error::RecvError::Lagged(n)) => {
                 warn!(skipped = n, "event handler lagged, missed events");