31: redesign block nodes, icons, and resize handle

[evgheni7]

Closes #31.

Summary

• Redesign block (resource) and container nodes with header actions and block-type icons
• Inherit block-type icon from provider so loaded blocks show it; show provider icon on accordion header instead of on individual block cards
• Add contrasting block card background for visibility
• Load provider metadata into provider block types
• Add fallback Box icon for blocks and accordion headers without an icon
• Replace tiny default resize handle with a standard bottom-right corner indicator

Test plan

• Open sidebar — accordion headers show provider icon or fallback Box
• Drop a block onto the canvas — block node shows icon, title, and Plus/Trash actions
• Drop a composition — container node shows icon, name, Plus/Pencil/Trash actions, and a visible resize corner inside the bottom-right
• Drag the resize corner to resize the container