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.

assets/icons/blocks/preview_size.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="24" height="24" fill="none" stroke-linecap="round" stroke-linejoin="round">
+  <rect x="3" y="3" width="7" height="7" rx="1" stroke="#cccccc" stroke-width="1.4"/>
+  <rect x="12.5" y="3" width="8.5" height="7" rx="1" stroke="#cccccc" stroke-width="1.4"/>
+  <rect x="3" y="12.5" width="18" height="8.5" rx="1" stroke="#4a9eff" stroke-width="1.4"/>
+</svg>