feat(insert): add docked block palette

Add a searchable block palette with previews, file import, collision-safe nested definitions, and dependency/table-record import. Keep palette thumbnails current across drawing switches and block-definition edits.

src/ui/window/mod.rs

@@ -1,4 +1,5 @@
 pub mod about;
+pub mod block_palette;
 pub mod layout_manager;
 pub mod layer_state_manager;
 pub mod plot;