Vim mode and cursor position moved into a shared store, and the cursor is restored across renders

A new module-level store holds vimMode and savedCursorOffset, initialized to vimMode: "INSERT" and savedCursorOffset: null. The input component seeds its cursor from the saved offset, saves it back on unmount, and re-clamps it in NORMAL mode using NFC-normalized text. Vim mode is forced back to INSERT whenever vim mode is disabled.

Returning from an external editor also re-clamps the cursor when in NORMAL mode.