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/MainView.tsx

@@ -28,6 +28,7 @@ import { HousekeepingView } from './housekeeping/HousekeepingView';
 import { RareValuesView } from './rare-values/RareValuesView';
 import { FortuneWheelView } from './fortune-wheel/FortuneWheelView';
 import { SoundboardView } from './soundboard/SoundboardView';
+import { ThemeApplier } from './theme/ThemeApplier';
 import { RadioView } from './radio/RadioView';
 import { InventoryView } from './inventory/InventoryView';
 import { ModToolsView } from './mod-tools/ModToolsView';
@@ -134,6 +135,7 @@ export const MainView: FC<{}> = props =>
 
     return (
         <>
+            <ThemeApplier />
             <div className="hidden" data-localization-version={ localizationVersion } />
             <AnimatePresence>
                 { landingViewVisible &&