Break the 3,244-line monolithic store into a Zustand slice-per-feature
layout (types, helpers, librarySlice, gallerySlice, searchSlice,
exploreSlice, albumSlice, duplicateSlice, taggerSlice, captionSlice,
settingsSlice, appSlice, events) combined in index.ts. Components keep
calling useGalleryStore(s => s.field) against the same flat state
object — no component changes required. Completes the de-godify effort
started with the menu/Dropdown/Sidebar extractions and icons.tsx.
