feat(slideshow): public fullscreen slideshow viewer

- /gallery/:slug/show/:token route + SlideshowPage: splash -> fullscreen kiosk,
  crossfade/cut/slide/kenburns/dip-to-white/dip-to-black transitions, color
  filters, white/original logo watermark overlay, contain/letterbox, cursor
  auto-hide, quiet-append of new uploads, live settings poll, and decode-ahead
  preload (first slide decoded before playback) so transitions do not struggle.
- slideshow.service for session/state + shared style types.

frontend/src/App.tsx

@@ -10,6 +10,7 @@ import { ThemeProvider } from './contexts/ThemeContext';
 import { GalleryPage } from './pages/GalleryPage';
 import { ClientAccessPage } from './pages/ClientAccessPage';
 import { PreviewPage } from './pages/gallery/PreviewPage';
+const SlideshowPage = lazy(() => import('./pages/gallery/SlideshowPage').then((m) => ({ default: m.SlideshowPage })));
 import { LegalPage } from './pages/public/LegalPage';
 import {
   AdminLoginPage,
@@ -159,6 +160,13 @@ function App() {
                   <Routes>
                   {/* Public gallery routes */}
                   <Route path="/gallery/preview" element={<PreviewPage />} />
+                  {/* Live Slideshow ("Diashow") — token-only fullscreen kiosk.
+                      Self-manages its session token; no GalleryAuthProvider. */}
+                  <Route path="/gallery/:slug/show/:token" element={
+                    <Suspense fallback={<Loading />}>
+                      <SlideshowPage />
+                    </Suspense>
+                  } />
                   <Route path="/gallery/:slug/client-access" element={
                     <GalleryAuthProvider>
                       <ClientAccessPage />