Add slideshow tab to gallery with duration control

[Realystic1]

Motivation

• Provide a slideshow view for a board so users can play images as a slideshow and control how long each image displays.
• Persist a per-user slideshow duration setting in gallery state so the UI and future slideshow playback can consume it.

Description

• Add a new slideshow gallery view and a slideshowDurationSeconds numeric field (default 5, min 1) to the gallery state and types (features/gallery/store/types.ts, features/gallery/store/gallerySlice.ts).
• Expose a selector and action for the slideshow duration (selectSlideshowDurationSeconds, slideshowDurationSecondsChanged) and wire them into the gallery slice and selectors (features/gallery/store/gallerySelectors.ts, features/gallery/store/gallerySlice.ts).
• Add a new GallerySlideshowPanel component with a NumberInput to edit duration, and integrate a Slideshow tab into GalleryPanel with conditional rendering of the slideshow panel (features/gallery/components/Slideshow/GallerySlideshowPanel.tsx, features/gallery/components/GalleryPanel.tsx).
• Add English localization strings for the slideshow tab and the duration label and update gallery localization (public/locales/en.json).

Testing

• No automated tests were run for these changes.

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Codex Task