Add a split tool for touch editing

[koharachan]

Summary

• add a dedicated split tool to the top toolbar for touch and pen editing
• show it only while split is available in select mode on touch/mobile contexts
• keep touch feedback stable even if the operation mutates selection state while running

Testing

• npm run test:once -- test/spec/ui/tools/split.js
• npx eslint modules/ui/tools/split.js modules/ui/top_toolbar.js modules/ui/tools/index.ts test/spec/ui/tools/split.js
• git diff --check

[matkoniecz]

add a dedicated split tool to the top toolbar for touch and pen editing

please post screenshot of this in action

[koharachan]