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/client-esp32/src/tasks/render.rs

@@ -44,11 +44,12 @@ pub fn run(
                 let is_screen_update = matches!(msg, ServerMessage::ScreenUpdate { .. });
                 let repaints = app.handle_message(msg);
 
-                if app.take_theme_changed() {
+                let theme_changed = app.take_theme_changed();
+                if theme_changed {
                     engine.set_theme(app.theme().clone());
                 }
 
-                if !repaints.is_empty() && (first_update || is_screen_update) {
+                if !repaints.is_empty() && (first_update || is_screen_update || theme_changed) {
                     let bg = engine.theme().background;
                     display.fill_rect(screen, bg).unwrap();
                     first_update = false;