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/api/utils/LocalStorageKeys.ts

@@ -5,4 +5,6 @@ export class LocalStorageKeys
     public static CHAT_WINDOW_ENABLED: string = 'chatWindowEnabled';
     public static CHAT_TRANSLATION_SETTINGS: string = 'chatTranslationSettings';
     public static CATALOG_CLASSIC_STYLE: string = 'catalogClassicStyle';
+    public static THEME_ACTIVE: string = 'nitroThemeActive';
+    public static THEME_PIECES: string = 'nitroThemePieces';
 }