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)

custom-themes/neon-viola/cards.css

@@ -0,0 +1,24 @@
+/* Tema Neon Viola — pezzo "cards" (finestre / NitroCard).
+   Ricolora header + cornice delle finestre. Caricato DOPO il CSS base, quindi
+   usa !important per vincere. Tocca solo la cornice/header (non lo sfondo del
+   contenuto) per non rovinare la leggibilita' del testo. */
+
+.nitro-card-shell:not(.nitro-wired) {
+    border-color: #7c3aed !important;
+    box-shadow: 0 0 14px rgba(124, 58, 237, .55), 0 8px 22px rgba(0, 0, 0, .4) !important;
+}
+
+.nitro-card-shell:not(.nitro-wired) .nitro-card-header-shell {
+    background: linear-gradient(180deg, #9333ea 0%, #6d28d9 100%) !important;
+    border-color: #a855f7 !important;
+    border-bottom-color: #2a0a4a !important;
+}
+
+.nitro-card-shell:not(.nitro-wired) .nitro-card-title {
+    color: #fff !important;
+    text-shadow: 0 0 6px #c084fc, 0 1px 0 #3b0764 !important;
+}
+
+.nitro-card-shell:not(.nitro-wired) .nitro-card-tabs-shell .nitro-card-tab-item-active {
+    box-shadow: inset 0 -2px 0 #a855f7 !important;
+}