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-domain/tests/render_tree_tests.rs

@@ -1,5 +1,7 @@
 use client_domain::{BoundingBox, LayoutEngine};
-use domain::{AlignItems, ContainerNode, Direction, JustifyContent, LayoutChild, LayoutNode, Sizing};
+use domain::{
+    AlignItems, ContainerNode, Direction, JustifyContent, LayoutChild, LayoutNode, Sizing,
+};
 
 fn screen() -> BoundingBox {
     BoundingBox::screen(240, 320)