implement pixel undos (fixes #5)

packages/client/src/components/App.tsx

packages/client/src/components/Toolbar/CanvasMeta.tsx

packages/client/src/components/Toolbar/Palette.scss

packages/client/src/components/Toolbar/Palette.tsx

packages/client/src/components/Toolbar/ToolbarWrapper.tsx

packages/client/src/components/Toolbar/UndoButton.tsx

packages/client/src/contexts/AppContext.tsx

packages/client/src/lib/canvas.ts

packages/client/src/lib/network.ts

packages/client/src/style.scss

packages/lib/src/net.ts

packages/server/prisma/dbml/schema.dbml

...r/prisma/migrations/20240427225450_add_undo/migration.sql

packages/server/prisma/schema.prisma

packages/server/src/lib/Canvas.ts

packages/server/src/lib/SocketServer.ts

packages/server/src/models/User.ts