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).

spa/src/components/app-shell.tsx

@@ -20,6 +20,7 @@ import {
   Database,
   Box,
   Layers,
+  Palette,
   Save,
   BookOpen,
   LogOut,
@@ -30,6 +31,7 @@ const NAV = [
   { to: "/data-sources", label: "Data Sources", icon: Database },
   { to: "/widgets", label: "Widgets", icon: Box },
   { to: "/layout", label: "Layout", icon: Layers },
+  { to: "/theme", label: "Theme", icon: Palette },
   { to: "/presets", label: "Presets", icon: Save },
   { to: "/guide", label: "Guide", icon: BookOpen },
 ] as const