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fix: remove unused GenericImageView import from board.rs extract_palette"

fix: board.rs - remove unused import warning (image::GenericImageView)

fix: real board files, active board state, drag targets, palette extraction from images

fix: board.rs - make board_add_image accept Option<String> for library_id to support internal calls

fix: implement real boards and usable board drag workflow

Add persistence session download stubs for Phase 3 alpha

Stabilize Phase 3 compile fixes and sidebar shield UI

Fix Shield popover airspace and add Phase 3 library board core