feat(theme): runtime custom theme ecosystem (graphics-only)

Runtime-loaded visual re-skin system (no client rebuild, real themes never
hit git). A theme = a folder on the server (theme.base.url) with a manifest +
CSS "pieces"; each piece is toggled from Settings > Themes (checkboxes). A
broken/404 piece auto-falls back to the default (per piece). Hotel-wide default
via ui-config theme.default (+ theme.default.pieces), per-user override in
localStorage (same pattern as the catalog style toggle).

- api/theme/ThemeManager: fetch index/manifest + inject/remove <link> + fallback
- hooks/theme/useThemes: state + persist + default-from-config + live apply
- components/theme/ThemeApplier: applies on boot (mounted in MainView)
- UserSettings: General/Themes tabs with theme selector + per-piece checkboxes
- custom-themes/: reference template (demo theme "Neon Viola" + README)
- .gitignore: public/custom-themes/ (real themes are never committed)

src/components/theme/ThemeApplier.tsx

@@ -0,0 +1,12 @@
+import { FC } from 'react';
+import { useThemes } from '../../hooks';
+
+// Mounted once at app level: subscribing to the shared theme store triggers the
+// load + apply effects, so the saved/default custom theme is applied on boot
+// and kept in sync when the user changes it from Settings. Renders nothing.
+export const ThemeApplier: FC<{}> = () =>
+{
+    useThemes();
+
+    return null;
+};