fix(undo): atomic batch undo for multi-shape operations

[eugenioenko]

Summary

• Add "batch" CommandType that wraps multiple sub-commands into a single undo/redo step
• Multi-delete, multi-paste, and duplicate now use batch commands so Ctrl+Z reverses the entire operation at once
• Batch inverse commands reverse sub-command order for correct undo semantics
• Single-shape operations still use individual commands (no overhead)

Test plan

• pnpm build passes
• pnpm lint passes (0 errors)
• New e2e test: draw 3 shapes, select all, delete, single Ctrl+Z restores all 3, single Ctrl+Shift+Z re-deletes all 3
• Manual: multi-select 2+ shapes, delete, verify single undo restores all
• Manual: copy 2+ shapes, paste, verify single undo removes all pasted
• Manual: select 2+ shapes, Ctrl+D to duplicate, verify single undo removes all duplicates

Fixes #46

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