feat(config): add mode color settings

[lajarre]

summary

• add piVim.modeColors for INSERT, NORMAL, and EX labels
• add opt-in piVim.syncBorderColorWithMode for editor borders
• keep existing ModalEditor constructor and clipboard-policy exports compatible

tests

• npm run check
• npm run pack:check
• node --import tsx/esm script/image-attachments-e2e.ts