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_engine_tests.rs

@@ -1,6 +1,6 @@
 use client_domain::{
-    BoundingBox, Color, DrawCommand, FontMetrics, FontSize, HAlign, RenderEngine,
-    ThemeConfig, VAlign,
+    BoundingBox, Color, DrawCommand, FontMetrics, FontSize, HAlign, RenderEngine, ThemeConfig,
+    VAlign,
 };
 
 fn metrics() -> FontMetrics {