feat: add undo/redo system (Ctrl+Z/Y)

[JWBWork]

Summary

• Adds a command-pattern undo/redo stack with batch support for compound operations (e.g. multi-file drag-drop)
• Tracks all canvas mutations: image add/delete/move/transform, polygon add/delete/move, vertex/midpoint drag, texture drag/transform/flip, atlas auto-pack
• Uses stage-level drag events for reliable tracking through Konva's Transformer

Test plan

• Ctrl+Z undoes each action type (add image, move image, add rect, move rect, drag vertex, etc.)
• Ctrl+Shift+Z / Ctrl+Y redoes
• Undo works while an object is still selected (transformer visible)
• Dropping multiple images at once undoes as a single action

🤖 Generated with Claude Code

[JWBWork]

heads up, we might want to merge this after other open feature branches

at least one of the other branches I opened would benefit from an undo function. probably best to merge those in and
I'll update this PR to make sure ctrl-z functions with those other features