[PB-6390] feature/Selection logic and actions sheet

[CandelR]

Implemented photo selection mode with toolbar and contextual action sheet to the Photos screen

Summary

• usePhotoSelection: new hook managing select state
• SelectionHeader: replaces the normal header when select mode is active
• SelectionToolbar: animated bottom bar that appears when items are selected
• MoreActionsBottomSheet: sheet with contextual actions filtered by whether all selected items are backed (Favorites hidden pending implementation in near future)
• PhotosScreen: wires selection state into the timeline, hides the tab bar while in select mode and restores it on exit