69367b45be
- New `slideshow` feature flag (backend KNOWN_FLAGS/DEFAULT_FLAGS, frontend FeatureKey + context default, a toggle card under Settings -> Features -> Core). Default off; strictly opt-in. - Move the global watermark defaults off the Event Types page into a dedicated Settings -> Slideshow tab (new SlideshowSettingsPage), shown only when the flag is on. - Gate the per-event Live Slideshow card and the per-event-type preset section behind the flag too (and stop writing a type preset when it's off). - en/de strings for the feature card + settings tab.